(TMI warning: for short version, skip to bottom!)
I'm 49.
I was never athletic in High School, though a couple of friends and I got together after school for maybe two weeks(!) to start an exercise program...I didn't even remember it until one of those friends reminded me of it!. And I don't even remember what we did.. maybe just dance around to music? That definitely doesn't count!
When I was an undergrad, I became more interested in health (went veggie in 1976, started working at a whole-foods co-op) and dabbled a bit in yoga (with some 30-day yoga program that I never finished), running (in bad shoes and up and down hills every day---this was the 'no pain, no gain' 70's-- which was the beginning of knee problems) and even a tiny bit of weight training (I picked up a weight set, after discovering that I had just easily lifted TWO 50-pound bags at the same time out of a truck that brought supplies to a co-op I was workig at, but had no idea what to do). I think I may have done more harm than good at that time, so luckily didn't continue.
In grad school (around 1983), I finally got more into working out. I biked a lot, did a century ride (finished the whole thing, but rather slowly!), and took a weight-training class. One day, when the regular teacher was gone because he was off with some sports team at a meet, a woman who was subbing for him corrected all our form (seems regular teacher didn't really 'teach' us much beyond book learnin,' and we were doing some things that could have caused injuries--I will always be thankful to that woman, whoever she is, and whom I never saw again). After figuring out that the regular teacher was not the best source of info, I picked up some books by Rachel McLish, Gladys Portuguese and 'Ahhhnold' and started reading things on women's bodybuilding (female bodybuilders back in those days looked buff AND feminine, unlike the 'men with long hair' look of the Ms. Olympia contestants of today).
I count this as my true beginning of my fitness quest (1983+), though there have been times since, like the first two years of my teaching, when I didn't exercise, but was shocked back into it when I weighed myself after two years and found I'd gained 10 pounds! That's when I started using workout videos (1992).
The short version: I've been working out seriously, with a few unfortunate long breaks, since 1992--14 years.