Trust me, from experience: you are not substantially different from school leavers at 21. You're all still basically in the same stage of your life (you might make slightly fewer bad decisions granted). The differences between mature students and school leavers tends to be more evident for mature students in their late 20s or 30 and above, which is usually not anything structural but just simply that by that age often those mature students already have existing friends/family/other commitments outside uni and so may just see the course as a stepping stone and not really interested in "the uni experience" or "making friends for life". Equally though for those who aren't in that bracket and do want to engage with uni in the same manner as a school leaver there's never an issue with that and "younger" students don't have any problem socialising outside their ages.
Bear in mind that uni is not like school - you aren't segregated by age. You aren't even segregated by subject! In many degrees on a given module (especially optional modules) you'll have a range of students from different degree programmes, some undergrad and some postgrad, some on exchange/visiting students, some students at your uni, all of varying ages and backgrounds. So invariably students mix across the range simply because they can (whereas in school due to not having lessons with older/younger students, you may have been less likely to hang out with them).