This was how it was in the ‘60s and ’70's in the UK.
At junior school we were never required to shower after PE, and changing for swimming at the local pool was done in a cabinet with your best friend, so there was no real exposure of the body.
That changed at senior school (11yrs+). There you showered after every sport, and the showers were open so you saw everything everytime, but you got so used to it you never really ‘saw’ anything. At times there would be mixed ages (11-19) using the showers, particularly in the pool, so you got to see all stages of development if you looked. You also had to strip off and wring out your trunks before moving in to the ‘dry’ changing area (A bit like using pools in Iceland). Again it all seemed 'normal'.
From the age of 12, I was also part of a swimming club and the changing areas were always open and we had all ages up to mature adults of my parents age and beyond using them together. Fortunately they were always single sex and no big deal. As it was the ‘70s, I did get to see my first ‘fully furred’ adult - I never knew you could have so much body hair!
In halls at college, the showers were communal, but the hall was single sex (We had a protocol agreed if a girlfriend wanted to use or entered the showers).
These days, the male changing areas of many sports venues I use are still mainly open, with a limited number of cabinets for those who'd prefer to use them. The showers are usually individual, but not everyone covers with a towel to and from.