The first time I ever tried it I must have been about 15. I'd always been really curious about circumcised guys, and I wanted to experience what it would be like to have no skin covering the head of my dick, so I decided to pull it back in my underwear and leave it that way for a week. At first it was incredibly uncomfortable, but by the end of the week it was tolerable, but still I was happy to stop. Sometime after I decided to try it again, but this time go for two weeks. It started off feeling tolerable yet still strange, and by the end of the two weeks I was completely free from feeling any chafing discomfort from my underwear, but I saw no reason to continue and stopped again.
A little while later I was on a family holiday in Southern Europe, and the weather was so hot that I decided to sleep naked one night, and the idea suddenly struck me that I should try sleeping naked with my foreskin pulled back - something which I'd never tried during my previous times retracting. I pulled it back, got into bed, and immediately got an erection, leaving the thin sheet draped over the bare head of my cock. It was a fascinating sensation, but it only made matters worse, and for hours I lay there unable to sleep with a throbbing teenage erection which wouldn't subside. The next morning I awoke with the skin still pulled back, but it was all pink, puffy, and horribly swollen, which rather alarmed me, and had me worried for the rest of the day.
After that I didn't do much retracting again until my 20s, when I started experimenting with doing it fairly regularly, for months at a time.