Glasgow has "changing villages", ie lots of cubicles by the poolside, some of which are family sized (not enoguh actually). Ds occasionally asks to use a cubicle on his own, (especailly if we are having difficulty getting a faimly cubicle), which I allow, but make it a race to see who gets changed quickest!
ds is 5 (6 in September) and I let him go to the loo now on his own if he insists. I do still stand outside the door worrying if he takes too long though! He loves the sense of independence and I think it iis good thing to encourage - and at the same time, to teach them about "stranger danger" - that no-one should ask them to do anything "secret" and that they weren't comfortable with.
Like Roisin, wouldn't use a public loo that I don't know (silly thing to say - if course I dont' "know" any public gents loos! )/ didn't like the lok of,
but ds has been in to Gents on his own at Pizza Express and various other restaurants (jncluding McDonalds), John Lewis, Homebase, Glasgow Aiport - and at the swimming pool. And he went off cheerfully to the loo on his at our hotel in Greece.
But then, I am definitely in the "ultra relaxed" school of parenting and allow ds all sorts fo freedoms (that I enjoyed when I was young) that some Mumsnetters would be horrified at!