How old are you?

George walks into my office. I ask him his age.

"I'm 21 years old," he declares.

Yet I look at George. He's got gray thinning hair, his posture is slumped forward rather than erect, the flesh on his upper arms hangs loosely, he's got wrinkles on his hands and face, brown spots on the back of his hands and arms. He looks more like 70 years old to me. "I don't think you're 21 years old. I think you're 70."

"Prove it," he says.

Okay. What now? Minus any formal identification like a driver's license, how do I prove that George is really 70-something and not 20-something? Not an easy thing, when you think about it. If George were a tree, I'd cut him down and count his rings. Is there such a phenomenon in humans?

This is actually a fascinating area of research, looking for reliable biomarkers of aging.

Among the most quantitative markers of aging is telomere length. Telomeres were once dismissed as nonsense sequences in DNA. However, more recent thought among geneticists is that telomeres shorten with aging and provide the body's cells a timeline of aging. This way, George's cells act like they are 70, not 13, and don't start producing gobs of growth hormone and testosterone in preparation for puberty.

What can slow or stall the shortening of telomere length? There are two I'm aware of:

1) Caloric deprivation--i.e., taking in fewer calories. This was among the theories explored by Dr. Roy Walford during his Biosphere2 experience, based on his work in mice that showed that caloric deprivation nearly doubled lifespan.

2) Vitamin D--Richards et al (2007) found that, the higher the vitamin D, the longer the telomere length. The highest vitamin D levels conferred a 5-year effective difference in telomere length.

So, if I could look inside George's cells and count his telomeres, I could judge with confidence whether he was 21 or 70. Or, he could take vitamin D sufficient to increase blood levels to a healthy range and be more like 65.