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For 93-year-old Joseph Brown, the clearest sign of aging was his inability one day to remember he had to have his pants unzipped to pull them on.

For 95-year-old Caroline Mayer, it was deciding at age 80 to put away her skis, after two hip replacements.

And for 56-year-old Dr. Thomas Gill, a geriatrics ­professor at Yale University, it's accepting that his daily 5 ½-mile jog now takes him upward of 50 minutes — though he long prided himself on running the distance in well under that time.

Is there such a thing as normal aging?

The physiological changes that occur with aging are not abrupt, Gill said. They happen across a continuum as the reserve capacity in almost every organ system declines, he said. "Think of it, crudely, as a fuel tank in a car," Gill said. "As you age, that reserve of fuel is diminished."

Drawing on their decades of practice along with the ­latest medical data, Gill and three geriatric experts agreed to help identify examples of what are often — but not always — considered to be signs of normal aging for people who practice good health habits and get recommended preventive care.

The 50s: Stamina declines

Gill recognizes that he hit his peak as a runner in his 30s and that his muscle mass peaked somewhere in his 20s. Since then, he said, his cardiovascular function and endurance have slowly decreased. He's the first to admit that his loss of stamina has accelerated in his 50s.

In your 50s, it starts to take a bit longer to bounce back from injuries or illnesses, said Stephen Kritchevsky, 57, an epidemiologist and co-director of the J. Paul Sticht Center for Healthy Aging and Alzheimer's Prevention at Wake Forest University. While our muscles have strong regenerative capacity, many of our organs and tissues can only decline, he said.

There can be a slight cognitive slowdown in your 50s, too, Kritchevsky said. As a specialist in a profession that demands mental acuity, he said, "I feel I can't spin quite as many plates at the same time as I used to."

The 60s: Susceptibility rises

There's a good reason why even healthy people age 65 and older are strongly encouraged to get vaccines for flu, pneumonia and shingles: Susceptibility and negative response to these diseases increase with age.

Hearing loss is common, Kritchevsky said, especially for men.

Reaching age 60 can be emotionally trying for some, as it was for Dr. David Reuben, chief of the geriatrics division at UCLA, who recalls 60 "was a very tough birthday for me. Reflection and self-doubt is pretty common in your 60s," he said.

The odds of suffering some form of dementia doubles every five years beginning at age 65, Gill said, citing an American Journal of Public Health report. While it's hardly dementia, he said, people in their 60s might begin to recognize a slowing of information retrieval.

The 70s: Chronic conditions

Many people in their mid-70s function as people did in their mid-60s just a generation ago, Gill said. But this is the age when chronic conditions — like hypertension or diabetes or dementia — often take hold. "A small percentage of people will enter their 70s without a chronic condition or without having some experiences with serious illness," he said.

The 70s are the pivotal decade for physical functioning, Kritchevsky said. Toward the end of their 70s, many people start to lose height, strength and weight. Some people report problems with mobility, he said, as they develop issues in their hips, knees or feet.

Another problem common to the 70s: People tend to take more medications used for "preventive" reasons. But these medications are likely to have side effects on their own or in combination, Gill said.

Perhaps the biggest emotional impact of reaching age 70 is figuring out what to do with your time.

The 80s: Fear of falling

Fear of falling — and the emotional and physical blowback from a fall — are part of turning 80. If you are in your 80s and living at home, the chance you might fall in a given year becomes more likely, Kritchevsky said. About 40 percent of people 65 and up who are living at home will fall at least once a year, and about 1 in 40 of them will be hospitalized, he said, citing a UCLA study.

The 90s: Relying on others

By age 90, people have roughly a 1-in-3 chance of exhibiting signs of dementia caused by Alzheimer's disease, Gill said, citing a Rush Institute for Healthy Aging study. The best strategy to fight dementia isn't mental activity but at least 150 minutes per week of "moderate" physical activity, he said. It can be as simple as brisk walking.

At the same time, most older people — even into their 90s and beyond — seem to be more satisfied with their lives than are younger people, said Kritchevsky.