My first experience was at secondary school where we were all required to take a shower in a very crowded changing room after PE. I was very self conscious being naked in front of others in those days and always worried about getting an erection. Happily I got more comfortable as time passed. Communal open showers are still common at many gyms and pools in the UK. I have noticed some of the younger guys showering in their underwear and carefully getting dressed under a towel. Others don't shower at all as has been widely noted. I'm glad that I worked through my own inhibitions and I now enjoy the experience of showering with others including mixed sex showers at the nudist swim group.
Most of my life. In Africa British grade schools all communal showers. In South America again after Sports gym or playing on a team again same thing communal showers. In a boarding High school 8 shower heads everyone naked and showering. No one said anything other than perhaps a tease like other people mentioned. Playing on a Varsity team in College again Communal showers. .
My first experience in an open communal shower was probably when I was 11 years old.
I was at a Boy Scout summer camp. All of the Scouts showered together. At first I was very nervous but after a few days, it was not a big deal at all. I remember looking around to see which of my friends were also going through puberty. None of the adult leaders showered with us but they were looking in to make sure nothing inappropriate was going on.
At the town pool, when I was seven or eight, with my dad. Since my folks were casual about bathroom nudity at home, I didn't find it strange. What I did find strange was that the crowd in the locker room was about half black. I hadn't noticed this in the pool itself for some reason. My school was about a fifth black, and I was un-used to being so nearly outnumbered. I later learned that this was because I lived in a racially-tolerant old underground railroad town (London, Ohio) and ours was the only public pool for many counties around that allowed black swimmers - so of course people came in from all over.