I started uni when I was 21 and I stayed in Hillhead. I'd say 80% were 18-19 year olds, 10% 20-21 and the other 10% 22 or older.
I don't know what it'll be like with corona this year, but last year Hillhead was full of 18/19 year olds partying all day and all night, so if you feel like you've grown out of that, maybe it's not the best spot.
But either way, there's so many people around that you're bound to find someone similar to you, and if you don't want to party you can just stay in.
The other student accommodations in Aberdeen like Unite or Liberty are good options as well, but I feel like you only meet 1/3 of the people you would meet at Hillhead.