Fifty years ago, Kathrine Switzer became the first woman to complete the Boston Marathon as an official entrant.

Last month, she ran the race again at age 70.

This time, though, she didn’t have to hide her gender from race officials by entering the race as “K.V. Switzer.”

This milestone shows how much progress women have made in sports in half a century. But it also marks another important feat — the enduring power of the “ageless athlete.”

Switzer even completed this year’s race just a little slower than her original run, in 4 hours, 44 minutes, and 31 seconds.

She is not alone.

You don’t have to look far to find athletes in their 70s, 80s, and 90s competing in a wide range of sports — triathlons, CrossFit, cycling, swimming, kiteboarding, mountaineering, and yoga.

And as lifespans grow longer, so do athletes’ careers.

A study last year in the journal Age & Ageing focused on the amazing performance of centenarian athletes.

Read more: Exercising in your 40s, 50s, 60s, and beyond »

Age-related performance declines

Athletes may be able to compete for decades, but they are not immune to the effects of aging.

“In general, performance declines linearly at about 5 to 10 percent per decade from age 30 years to around 60 to 65 years — less of a decline in those master athletes who train hard — after which the decline accelerates,” Peter Reaburn, PhD, a professor of exercise and sports science at Bond University in Australia, told Healthline.

This can vary by activity.

“Performances in speed and power events appear to decline at a slightly faster rate than endurance events,” said Reaburn.

He added that this is “related to drops in muscle mass. Hence the importance of weight training to hold onto muscle mass.”

Athletes in nonweight-bearing sports, like cycling and swimming, may see smaller drops compared with sports that include weight-bearing exercises like running.

Several studies found that short- and long-distance triathletes saw smaller age-related declines in cycling performance than running and swimming performance.

Other factors are also affected by age.

“Starting in their early 30s, endurance athletes begin to experience a decline in some of the markers of fitness — aerobic capacity, [lactate] threshold, and economy,” said Joe Friel, an endurance sports coach, and author of several training books, including “Fast After 50: How to Race Strong for the Rest of Your Life.”

Research suggests that a decline in VO2max — the maximum volume of oxygen that an athlete can use — is the primary factor behind age-related drops in performance.

“Aerobic capacity — VO2max — declines at the rate of about 1 to 2 percent per year, depending on how the athlete trains,” said Reaburn.

Lactate threshold is the exercise intensity at which athletes can sustain high-intensity effort only for a short time.

Exercise economy — the efficiency at which athletes turn metabolic energy into mechanical power or velocity — “seems to be the most stable of the three markers,” said Friel.

Read more: Exercise plan for seniors »

Athletes aging gracefully

Older athletes are prone to many of the same problems that affect the performance of younger athletes — genetics, injuries, motivation.

But not at higher levels.

“Research suggests that aging does not cause an unusually high risk of injury,” said Friel, “since the body adapts throughout life to the stress and strains of the sport.”

That means declines are mainly due to the normal aging processes.

“In speed and power sports, it’s muscle mass and nervous system declines,” said Reaburn. “In endurance events, it’s the same, but also declines in the cardiovascular system — in particular, a decline in maximal heart rate and the ability to pump oxygen to working muscles.”

So you may not be able to prevent performance drops as you age, but you can minimize them.

“Use it or lose it, definitely,” said Reaburn.

A 2002 study of female endurance athletes found that declines in VO2max were linked to reductions in exercise training volume and intensity.

Athletes also need to choose their activities carefully.

“Long, slow distance training — which is what most are inclined to do as they get older — has been shown to reduce aerobic capacity relative to what it may be with high-intensity training,” said Friel.

To counteract this, Friel suggests that “older athletes generally need to ensure they are doing high-intensity sessions at least once or, better, twice weekly.”

Athletes can maintain their edge by continuing to train smart. But even if you are still sitting on the sidelines, it’s never too late to start.

Canadian athlete Ed Whitlock, who was the first person over 70 to run a marathon in less than three hours, took up running in his 40s.

If you’re wondering what sport is best for you to do as you age, follow your heart … or your feet.

“The most effective way to slow the declines of aging is to do what you most enjoy doing,” said Friel.

Read more: Olympic athletes: How in the world do they perform at such a high level? »