When I was 19 I worked in a place where most of the staff were 18-21, the managers and other supervisory staff were a fair bit older, one of them was early 30's and he always came out on nights out and things like that with us, he was a really nice bloke, and we didn't mind him at all, but still we did think it was a bit weird that he'd always come out with us - but then in that environment where most of your colleagues are a good 10+ years younger than you, I guess he didn't have too many options.
The situation is going to be similar for a lot of us at uni, I'll be 30 and I'm sure I will make friends with younger people, but I've no desire to be that guy. I'll be staying in halls, but with mature students. I can't really see myself making proper friendships with those straight out of A-Levels, as young at heart and as immature as I can be, our sensibilities and outlooks on life are just too different, and yeah that is a sweeping judgement before I've even met anyone, but I think I'd rather prefer to socialise with people a bit closer to my own age.
But each to their own, if you want to experience all the parties and what not OP, then go for it, noone's going to really question you for it, but there will be a lot of people your age as well, most unis have mature student societies etc.
tl;dr probably not best to have your social circle consisting entirely of 18 year olds, but noone is going to blink an eye at you for going to parties etc.