At the age of 65, Gene Avrett took up tennis.
Avrett, now 70, says he's functioning at the level that he was in his 50s.
Avrett, an astrophysicist at Harvard, believes his newfound energy comes from a bottle of synthetically-created human growth hormone.
"I think there's a definite health benefit," says Avrett. "I'm stronger and run around the tennis court faster."
Avrett's doctor, Ronald Livesey, started prescribing growth hormone after trying it himself eight years ago.
"I've noticed many changes," says Livesey. "My lean body mass has been restored, and my waist has gone from 39 inches to 32."
His patients pay as much as a $1,000 a month for so-called anti-aging treatments.
So why hasn't the medical community at large embraced this as a routine treatment?
"Everyone has a natural skepticism to this, and I think that's primarily because everything before this was done by snake oil salesmen and charlatans," says Livesey.
Human growth hormone is approved for use in patients whose pituitary glands don't make enough of it, including children who are destined to grow up to be extremely short. But what's being marketed is a theory not proven by any scientific studies -- that using growth hormone can actually undo the ravages of aging.
"The physicians, clinicians and anti aging clinics that are administering growth hormone to their patients before the clinical trials are, in my opinion, acting irresponsibly," says professor Jay Olshansky.
Olshansky studies aging, and he knows the power of growth hormone first hand.
"My son started using growth hormone when he was 8 years old," says Olshansky.
Olshansky's son Ricky suffered from growth hormone deficiency and wasn't expected to grow taller than five feet. But now, at 16, he's 5-foot, 7-inches tall.
"I would say growth hormone is one of those products that is dangerous, not because it doesn't do anything, but because it does something," says Olshansky.
There is preliminary evidence that adults taking growth hormone may be at higher risk for diabetes and even cancer.
But Avrett says it's a chance worth taking if it means he can stay one step ahead of the hands of time.