My experience from the IYNO message board:

I first became interested in it at about 11/12 - did swimming squad and the team would all shower naked and after the summer holidays my mate and I got put promoted from the kids to the teen squad which happened later in the afternoon. I was a bit nervous about it and asked my mate if he would shower nude with them and he was like yeh of course - we don't want them to think we are not part of the team, so just took the plunge and stripped off with everyone. It probably helped i had hit puberty cause the older swimmers were like wow look at this guy and made light hearted jokes and comments about everyone and just felt so natural to be nude in front of each other and not have to hide.

So later when i was about 15 had the chance to go to a nude beach and loved the feeling of being outside nude in the sun and water but wouldn't have called myslef a nudist until early 20s when started ]couchsurfing and stayed with a nudist in Paris who hosted a nude dinner party and blew me away the feeling of being able to be nude socially.