Age is said to bring wisdom and experience. But it can also bring some less desirable side effects: decreased energy, weight gain, loss of libido and slower brain function.
In women it's known as the "change of life," or menopause, when a woman's hormone levels drop precipitously, altering her body chemistry in ways that leave lasting results. Now, some doctors say it's not just women who experience menopause.
That's right -- male menopause, also known as andropause.
Watch the story tonight on "Nightline" at 11:35 p.m. EDT
It's different, to be sure. While women's estrogen levels drop relatively abruptly -- about 5 to 8 percent over the course of a year or so -- men's loss of testosterone happens more gradually at about 1 to 3 percent every year, starting in the mid-30s.
Some doctors say it's a condition that can be treated. Leading the way is Cenegenics Medical Institute, a Las Vegas clinic founded by Dr. Alan Mintz, who said he's found a way to help his 12,000 patients slow down the natural aging process. Mintz's clinic treats both men and women, but the vast majority of its clients are male.
"People come to us because they're concerned about the following four or five things: energy, body composition, libido or sexual function, cognitive function, and in some form, a weakened immune system -- they're getting sick more often," Mintz said.
"They may not be ill in the disease-based sense that we talk about, they may not have something wrong with them, but they know they're not the person they were 10 years ago. They're sick and tired of being sick and tired, and not getting answers. So we look, in a very comprehensive way, at why they're not feeling the way they did when they were 30 or 35 or 40. And there are real answers to be found in science."
That answer, he said, is the endocrine system: the complicated balance of hormones that changes as people age. The normal level of any given hormone exists on a sliding scale, and the doctors at Cenegenics say their intention is to evaluate their patients' current levels and then prescribe a regimen intended to boost those levels to the upper end of that normal range.
Patients travel to the clinic for an intensive daylong examination and personal history. Most leave with a prescription for one or more hormones, which they'll inject at home, along with recommendations on changes to their diets and lifestyles.
Mintz said most doctors have long ignored the importance of hormone levels to a patient's health.
"I try and look at the analogy of an orchestra playing Beethoven, 9th Symphony -- 100 instruments in the choir. You need to pay attention to all the instruments in the choir. And if we try to achieve this endocrine balance, and metabolic balance, we're healthier."
He was his own guinea pig for this controversial treatment. Mintz, once an overweight child, started running to lose weight and eventually become a marathoner.
But in his mid-30s he started noticing his body declining.
"I didn't like it, and the answers I was getting from my colleagues was, you're just getting old like the rest of us, Alan. Well, I didn't like that. I felt like I had more than half my life ahead of me, and I didn't want it to be this slide downhill."
A radiologist, he started reading about hormone therapy and put himself on a regimen.