Even though life expectancy is on the rise, Alzheimer's disease (AD) and dementia are at an all-time high. In its latest report, the Alzheimer's Association found that an estimated 5.2 million Americans had AD in 2014. The good news: Research suggests what you do during your free time—even decades before you reach your golden years—could help you stay sharp later in life.

Use Your Head: Protect Your Heart

What's good for your heart is good for your brain. Taking the following steps to keep your heart healthy may also help stave off cognitive decline:

• Don't smoke.
• Sleep 7-8 hours a night.
• Keep your blood pressure and cholesterol levels in check.
• Eat a low-fat, healthy diet.
• Get plenty of exercise.
• Maintain a healthy weight.
• Limit alcohol consumption.
• Get blood sugar levels (and diabetes, if you have it) under control.

In a survey of people in their 70s published in the March 2001 issue of the Proceedings of the National Academy of Sciences, those who regularly engaged in mentally challenging hobbies during their younger years were found to be 2.5 times less likely to develop AD than those who limited their leisure time to watching TV.

While stimulating your brain during the critical period from age 20 to 60 doesn't guarantee you'll remain free of AD, it could delay the disease for years, according to some experts.

Up to 40 percent of our risk for developing cognitive impairment can be attributed to lifestyle factors, many of which start before we're even born, according to Janet Jankowiak, MD, a geriatric and behavioral neurologist and a member of the American Academy of Neurology (AAN).

"People who have a family history of dementia should do everything they can to counterbalance those [risk] genes," Dr. Jankowiak says. That includes not smoking, controlling cardiovascular risk factors such as cholesterol and blood pressure, exercising regularly, and staying mentally active. The brain is similar to a muscle—you use it or lose it.

Researchers think engaging in stimulating activities during your younger years may build up cognitive reserve, the brain's ability to cope with increasing damage while still functioning adequately, explains Naomi Steiner, MD, a developmental and behavioral pediatrician in Boston, MA. Some experts believe that by building cognitive reserve, you can continue to function at your mental best into advanced age—and that if you do begin to experience cognitive decline, it may take longer for symptoms to develop.

Head Strong: How Experts Preserve Their Minds

Aging doesn't have to be a passive process. Experts agree you can intervene. "That doesn't mean you're never going to lose your memory, but if you lose it at 82 instead of 77, that's a big deal," says Ronald Petersen, MD, director of the Mayo Clinic Alzheimer's Disease Research Center in Rochester, MN, and member of the American Academy of Neurology (AAN).

We asked the experts what they do to stay sharp:

"I make exercise a priority. Whether I'm walking outdoors or visiting the health club at work, I make a point to build exercise into my daily routine." — RONALD PETERSEN, MD

"I swim a mile and a half three times a week and walk three miles a day. I'm also a fabric artist and I'm very active on the board of my local YMCA." — JANET JANKOWIAK, MD

"Instead of copying down medical record numbers, I look at the numbers, memorize them, and then write them down." — NAOMI STEINER, MD

Already in your golden years? Don't sink into the couch. While scientists have traditionally viewed brain cells as finite resources, they're now learning that the brain continues regenerating and forming new connections throughout one's life. Although most cognitive reserve is probably built up early in life, engaging in mentally stimulating activities at any age may have a positive effect—and it doesn't have any negative side effects.

More Research Needed—But don't Wait

While brain training activities—such as computer games that aim to improve memory and attention span—is an exciting area of research with no downside, none of these activities are proven buffers against AD.

"When a study shows an association between two things—such as mental stimulation and a decrease in risk of AD—people often make that next leap to assuming mental stimulation causes a decrease in AD risk," says Dr. Jankowiak. "But none of these studies definitively show cause and effect."

Nevertheless, experts agree evidence is building to support the idea that continually challenging your brain in healthy ways, spending time with loved ones, and having fun goes a long way in terms of safeguarding the brain. The key is to participate in activities you enjoy, according to AAN member Ronald Petersen, MD, director of the Mayo Clinic Alzheimer's Disease Research Center in Rochester, MN.

"We can't document that any of these activities are better than another, so don't do 10 crossword puzzles today if you hate crossword puzzles," Dr. Petersen says. "Taking care of your brain shouldn't be a chore, but rather a source of entertainment. That way, if they're fun to do, you have nothing to lose."

Here's your go-to guide to a few brain-boosting activities.

Learn a Second Language

Research suggests that people who learn a second or third language during childhood may be cognitively better off in the long run. A November 2013 study published in the AAN's medical journal Neurology found that learning a second language, and speaking it regularly, delayed the onset of dementia for 4.5 years on average. The benefits even held among people who were illiterate.

"We know that procedural memories such as riding a bike, or skills you learn at an early age, create pathways in the brain that are essentially permanent," says Dr. Jankowiak. Say you learn the word for "mother" in Spanish is "madre." Initially, you have to concentrate to remember that word, but if you continue studying Spanish, over time it will come to you seamlessly. That higher level of proficiency is evidence that your brain has made new connections and laid down a pathway so you don't have to concentrate to call up a memory. Creating more of these pathways may be the key to building cognitive reserve.

QUICK TIP: Pick one night a week to speak in another language (however poorly), or stay silent and communicate with sign language. Switching between different languages engages the brain. "If you aren't getting your message across one way, you have to take that phrase and come at it from a different direction," explains Dr. Jankowiak.

Become a Social Butterfly

Studies consistently show that socially active seniors are less likely to develop AD than their wallflower counterparts. Research also suggests strong social networks minimize the effect of AD-related brain damage on thinking and memory.

A study of 89 elderly people published in the journal Lancet Neurology, for example, reported that those with large social circles scored higher on cognitive tests (especially tests of working memory and language skills) than those who were more reclusive-even if they had the hallmark plaques and tangles of AD.

Researchers aren't sure how or why social networks may help keep the brain resilient against the hallmark pathology of AD. Some contend that the traits that enable people to build and maintain friendships act as a buffer against cognitive impairment.

"Regardless of the reason, we know people don't do well in isolation," says Dr. Steiner. "Making friends as you get older is especially important since it not only increases cognitive function, it also seems to ward off depression, which is not good for brain function." A bonus: When people are feeling good about their social networks, they tend to make healthier choices in other areas of their life.

QUICK TIP: Instead of simply joining the local senior center, volunteer for an organization that serves multiple generations. "Developing friendships with people in different age groups increases the challenge to your brain since it requires you to tailor your language to the appropriate age," explains Dr. Jankowiak.

Play Some Tunes

You don't need to play like Mozart to benefit from music. Dabbling in guitar or learning to play "Chopsticks" on a keyboard offers countless benefits.

Playing a musical instrument requires significant brain power, according to Dr. Steiner. You have to read and decode notes, coordinate movement with your fingers, and in many cases, use both hands. In fact, a growing body of research suggests the architecture of the brain changes in response to musical training.

In one study, non-musicians were assigned to perform a five-finger exercise on the piano for two hours a day. Within five days, the parts of the brain that control motor skills, hearing, and memory became larger and more active. A bonus: Listening to soothing tunes can help calm the mind and the body, which is good for the brain.

QUICK TIP: If you're having trouble finding your words, sing! In one recent study, researchers gave AD patients five words to remember-one group recited the words aloud three times, while the other had to sing the words to a tune. Perhaps not surprisingly, the people who broke out in song had better recall than those who spoke the words. Researchers suspect the emotion involved in singing activates areas in the brain that are responsible for encoding and retrieving information.

Get Moving

Research consistently links physical exercise with cognitive benefits. In a November 2011 review article published in the journal Neuroscience, researchers concluded that regular physical activity during early adulthood and midlife preserves the mind later in life. The study cites research showing that physical activity boosts brain function in regions associated with executive function, such as planning, scheduling, inhibition, and working memory.

A second study published in the January 2010 issue of the Journal of the American Medical Association-Neurology (formerly Archives of Neurology) found that any frequency of moderate physical exercise reduced the risk of developing mild cognitive impairment (MCI). Other studies have shown that the size of the hippocampus (the brain's center for memory formation) increases with exercise.

"Physical exercise may produce enzymes that help break down bad proteins like amyloid in the brain," says Dr. Petersen. "Exercise may also induce something called brain-derived neurotrophic factor (or BDNF), a protein that promotes the growth of new neurons."

Don't worry. You don't have to spend 90 minutes at the gym every day to reap the rewards. Even moderate exercise—about 150 minutes each week—can provide a huge health benefit. Jogging, cycling, swimming, walking, even ballroom dancing all get the blood flowing to your noggin.

QUICK TIP: Preliminary research suggests that combining physical activity with mentally stimulating activities may have a synergistic effect. So things like taking a dance class, learning martial arts or practicing yoga may be especially beneficial. The theory: Mental exercise enhances synaptic activity, while physical exercise increases blood flow to the brain.