Why are some people sharp as a tack at 95 years old, while others begin struggling with mental clarity in their 50s?
A lot of it has to do with genetics, but certain lifestyle factors also play an important role in how our brain ages. So while you can't control your genes, you can take advantage of the latest science and avoid these seven big brain mistakes:
Mistake No. 1: Eating a standard American diet
Foods high in sugar, unhealthy fats and processed foods -- i.e., the typical American diet -- can wreak havoc on your brain over time. Studies have shown that excess sugar consumption can impair learning and memory, and increase your vulnerability to neurodegenerative diseases like Alzheimer's. Some scientists have even referred to Alzheimer's as "Type 3 Diabetes," suggesting that diet may have some role in an individual's risk for developing the disease.
A Mediterranean-based diet, on the other hand, can help protect the brain from signs of aging and ward off cognitive decline. A recent study showed that following this type of diet -- which is a good source of brain-healthy nutrients and includes a lot of fish, healthy fats, whole grains and vegetables -- could slash Alzheimer's risk by up to 50 percent.
Mistake No. 2: Living next to a highway
Living in a smoggy city might be bad news for your brain. According to research published this month in the journal Stroke, exposure to air pollution is linked with premature aging of the brain.
The researchers found that people who lived closer to a major highway had greater markers of pollution in their lungs and blood, which increased their risk for a form of brain damage known as "silent strokes," or symptomless strokes. Increased pollution volume was also linked to decreased brain volume -- a major sign of aging.
Mistake No. 3: Drinking a few evening cocktails
Don Draper's daily cigarettes and two-martini lunches might seem glamorous on "Mad Men," but research suggests that they're a fast track to neurodegeneration.
It should come as no surprise that excessive drinking and cigarette smoking at any stage of life can have a negative effect on the brain, damaging brain tissue and leading to cognitive impairment. Alcoholism can cause or accelerate aging of the brain.
But just a couple of glasses of wine a night could pose a risk to brain health, even though there are some cardiovascular benefits. A 2012 Rutgers University study found that moderate to binge drinking -- drinking relatively lightly during the week and then more on the weekends -- can decrease adult brain cell production by 40 percent.
"In the short term there may not be any noticeable motor skills or overall functioning problems, but in the long term this type of behavior could have an adverse effect on learning and memory," one of the study's authors, Rutgers neuroscience graduate student Megan Anderson, said in a statement.
Mistake No. 4: Giving in to stress
Living a stressful lifestyle may be the worst thing you can do for your health as you age. Chronic stress is known to shorten the length of telomeres, the sequences at the end of DNA strands that help determine how fast (or slow) the cells in our body age. By shortening telomeres, stress can accelerate the onset of age-related health problems.
What about the brain? Well, some research has suggested that high levels of stress hormones can increase an individual's risk for age-related brain damage.
"Over the course of a lifetime, the effects of chronic stress can accumulate and become a risk factor for cognitive decline and Alzheimer's disease," Howard Fillit, a clinical professor of geriatric medicine at The Mount Sinai School of Medicine, wrote in Psychology Today. "Several studies have shown that stress, and particularly one's individual way of reacting to stress (the propensity to become 'dis-stressed' often found in neurotic people for example), increases the risk for Alzheimer's disease."
If you're feeling stressed out, try picking up a meditation practice. Research has shown that meditation is effective in lowering levels of the stress hormone cortisol and protecting the brain from aging.
Mistake No. 5: Getting by on less sleep than you need
There are a number of scary health effects associated with sleep deprivation, from a higher risk of stroke and diabetes to impaired cognitive functioning. Over the years, losing shut-eye can also accelerate brain aging. In a study conducted last year, researchers from Singapore found that the less that older adults slept, the faster their brains aged.
The study's lead author explained in a statement that among older adults, "sleeping less will increase the rate their brain ages and speed up the decline in their cognitive functions."
Mistake No. 6: Sitting all day
It's a well-established fact that sitting for long periods is terrible for your health. A growing body of research has linked a sedentary lifestyle with health risks including heart disease, diabetes, cancer and early death, even among people who get the recommended daily amount of exercise.
And it turns out that sitting is also pretty bad for your brain. Research has linked physical inactivity with cognitive decline. Moreover, weight gain in older adults -- which may result from too much sitting -- has been linked with shrinkage in brain areas associated with memory.
So when in doubt, move around. Physical activity has been linked with a number of brain health benefits, including improved learning and memory.
Mistake No. 7: Zoning out
Use it or lose it! If you want to keep your brain sharp, keep it engaged. It doesn't have to be a challenging intellectual task or a brain-training game, either -- simply engaging in everyday activities like reading, cooking or having a conversation (as opposed to vegging out in front of the TV or computer) can make a difference.
But mental exercises like crossword puzzles and sudoku can help, too. A 2013 study published in the Canadian Medical Association Journal found that brain exercises are more effective than drugs in preventing cognitive decline.
The bottom line? Doing new and novel things promotes neurogenesis, the creation of new neurons in the brain. So get outside, learn, discover and try something new to keep your brain sharp through the decades.