I did my undergrad at Sheffield, and am currently doing my postgrad at KCL. I'm also 22, straight out of my undergrad, but am living at home.
With regards to Sheffield specifically, there was a pretty clear undergrad/postgrad divide, but you'll probably be fine in something unrelated to your studies like cheerleading. If, as you said, you look the age, then I see no reason for why you might stick out. The subject specific societies were all usually undergrad dominated, so you probably won't have much luck there, but you'll be fine everywhere else (particularly in the smaller societies, where everyone is a lot more friendly and where it is easier to find familiar faces). Sheffield's SU is amazing - definitely take advantage of its societies, facilities, events etc. There is a reason why it has been voted the best SU in the UK for a number of years.
Most Sheffield postgrads lived in Allen Court. Endcliffe and Randmoor are almost entirely Freshers - you would definitely stick out there, and I certainly wouldn't enjoy it as a 22 year old. That said, you can opt for a postgraduate only flat in those villages - something tells me those postgraduates would be a little more up for socialising. Like I said, the smaller societies might be best, so if you have any quirky interests, give those a try. Cheerleading shouldn't be a problem, although it is quite big. Sport societies will usually have a small number of postgrads.
I met quite a few postgrads in Sheffield as a Residential mentor, and my experience of them was that their social life was as good as they wanted. The home students were typically less interested in going out, meeting people etc, whilst the international postgrads were almost as crazy in that regard as freshers. I suppose if you have already studied at Sheffield or have a friendship group nearby, there's no real motivation for making an effort with people that you might just see for the year. I would advise you go for all the events your department advertises for postgrads, and try to mingle with the internationals if the home student crowd on your course aren't very active.
With regards to workload, I can only really comment on my own. It is a lot, but if you manage your time well, you should be fine. The number and size of assessments have increased slightly, but the reading is about the same. Whereas during undergrad it's more concentrated towards the end of term, postgrad seems to be more spread out. So think of it as constant low-stress, rather than mostly relaxation until a high dose of stress at the end! This is all dependent on your course structure though. Socialising on my course has been quite good, but that might be because it is a very career-related course so people seem keener to make contacts.
I hope that helps. Feel free to reply or message me if you have any other q's!