I know quite a few students here that are 20-23 in Halls, and they've all seemed to fit in just fine. The age difference really doesn't mean so much, as in every hall you get the quieter, shyer 18-19 year olds anyway. In all honesty, I think the age gap only matters when it matters to you, I sort of know a guy who is about 27 and lived in Halls, and it was him that worried about the age gap all the time, not everyone else.
Which Halls are you applying for, out of interest?