It was a bit awkward at 19, a floormate of mine was 20 and he really felt the age difference. Some were 17! But when you apply as a mature student they try and group you with fellow older students. Well, some schools try. I'd personally aim for a student apartment.
you're 21 and that's definitely not too old to be in the dorms. it's probably the perfect age b/c you're old enough to buy liquor and party with real adults. I think when you start getting in that 25+ range it gets weird. No one will say "hey you're too old to be here!" but imo what happens is you will feel the age gap immediately between the 18 year olds idea of fun and yours. Or what was funny at 18, 19 isn't necessarily funny anymore at 25, 26, 27 etc. Again, you're not even in that 'too old for dorms' age group anyways.
I remember a lady who was 25 and still living in the dorms. I thought it was a little odd, because who would want to live in a dorm at that age? But I would say under 25 is definitely okay. And at 21 you will still make friends.
One of my relatives was in the dorms until he was 24 or so. He was working in a graduate program, and stayed in the dorms all through his undergraduate and graduate degree because he was offsetting his college costs by working as a dorm floor RA. He had plenty of friends and still stays in touch with them. I'm sure you'll feel kind of a disconnect with the 17 year olds who are fresh out of high school just because a lot of them will be kind of immature, but so will other students your age.
The dorms are a great place to make friends. It's the one thing I regret about not going to a four year, away from home college.