I don't know how it compared cos technically I didn't run all the way, but I started doing ultra-type stuff when I was 15, in the form of 50-mile, 24 hour challenge hikes (as a Venture Scout) in the Chiltern Hills. We just hiked them as fast as we could and our training was one long fast hike at the weekends and then just circuit-training and normal 15-year-old kid type stuff during the week, so no hard-core training but a lot of determination!
That first one I finished in about 18 hours (was a self-navigated course in teams of 4 through the winter hours of February). I was hooked and continued to do them into my late 20s before getting distracted by other stuff for a while. I'm not saying it was good or bad, just that I loved it even then.
So I guess I'd say go for it if you want to, but be really sensible about training and eating properly and not going OTT, especially as you want your body to last you a whole lot longer and hopefully keep doing this stuff. I'm now in my early 40s and back enjoying the joys of ultra-running. I have niggles for sure, but not ones I'd put down to my efforts of my youth.
Anyway, that's my thoughts ...
Edited: 21/02/2013 at 20:22