Hi, I returned to do Partiii Math at 50, had a really great time, got a first. I'm very anti-social anyway and I was there to study, as were most people, not to make friends as such, but had no problems talking to people or studying with them. (Hint: Krispy kreme works wonders to get 18yr olds out of bed for a study session at 8am on Saturday). Once, in the middle of winter term, someone to whom I'd never spoken (or even recognized) ambushed me as I exited the bathrooms to ask point blank: "I've seen you in many of my classes, and you seem older than the rest of us; what's your backstory?" I mean, no "Hi , I'm Bob" or "Hey, great weather today!" or even "Did you manage Q14b of the latest assignment?" or anything at all as a prelude. I thought it was really rude, but he was, after all, American. I bit my tongue and stopped myself from retorting "Hi, I've seen you in a lot of my classes, and you seem a lot stupider than the rest of us; what's your backstory?" and instead politely tried to answer his questions the best I could. Another time, there was a (much nicer) guy, and I was in the middle of some intense discussion about some HW problem with him, and as we exited the building into the rain, he put his umbrella up to cover me, which I thought was funny in a sweet way. Although, he wasn't European - maybe that was cultural.
I imagine I was the oldest person on the course (although not the stupidest) (which is really the more important marker). But if you want to talk about lack of diversity, I was much more conscious of the fact that, e.g., in one class of 20, I was the only F. I heard it's even worse in physics. Certain ethnic groups were very underrepresented, although, curiously, being blonde seemed also to be a rarity. It's not uncommon in the US for teenage mothers to attend lectures with their babies in prams - but I can't see that happening much here.
I found it irritatingly disrespectful that a few people would sit in the front row and then text or mess around on facebook throughout lecture, but I've seen this in many other settings - there are immature people at any age. You'll probably find that you have a big advantage in being focused on your purpose in being there, so you'll be a lot more efficient. You'll also be able to view things in context - e.g. it's not the end of life as you know it because someone you've been dating for three whole weeks (!) has dumped you - (seriously, look at some of the top trending threads on this board - and what kind of person averages 20+ posts a day on an internet chat forum anyway?!) - just roll your eyes, and realize these people (i call them "kids", not people, if they behave like that) are in competition for marks - so they can happily fill out the pass3 slots, while you get on with getting the job done and achieve your qualification. Having said that, the vast majority of students that I met were exceptionally bright and motivated and inspired, and I learned a great deal of math from them (in exchange for doughnuts). Age didn't really come into it.