I graduated last year but will be taking an MA at a different uni from this autumn. Having been through the 'do I?, don't I?' social life conundrum as an undergrad, I hope my experience helps reassure anyone who's feeling nervy about interacting/socialising with 'the youngies'.
1) Don't reveal your age until people know you (especially if you're over 30). It amazed me, but anyone under 25 is incapable of determining the age of anyone over 25 other than they are 'over 25'. This only applies if you don't try to dress like an 18-year-old, or like their mum/dad - or your mum/dad. My peer group decided I was around 15 years younger than I am - and I didn't advise them of the grisly truth until when into my 2nd year - by which time it made no difference.
2) The youngies will take their cue from you. If you treat them with respect then they'll return the favour. Expect to learn as much from them as they learn from you.
3) You need to make the effort to get to to know the youngies. They will assume that you wish to keep your distance unless you approach them. It's easier to cross the age divide and build relationships before attempting to drink them under the table so use Freshers to get to know those in your halls / on the same course / taking the same modules / other 'oldies' before scuttling into the SU when it's heaving with staggering 18-year-olds.
4) Any Freshers event (or any other campus event) which is alcohol-sodden is best attempted with a fellow 'oldie' as you're less likely to feel like a total div (regardless of how many youngies are with you).
5) Expect the youngies to come to you for guidance on all things academic / uni-related, but don't offer any advice about anything to do with popular culture (they are experts).
6) If you have the chance to be a student rep.... do it! This is one way that your 'maturity' can really benefit your fellow students (and they appreciate it). It will also give you lots of chances to get to know people across your course/school/faculty/whatever and provides lots of opportunities to socialise.
7) Expect to be hit-on repeatedly by younger students (this applies regardless of their age - or your age). Not sure if it applies to both genders but it's a current phenomenum which will catch-out the unsuspecting. Whatever the temptation.... if the age diff is 5+ years....DON'T do it! Your street cred will be permanently destroyed.
8) Don't get totally ratted in large student gatherings. DO get totally ratted in small gatherings where you know them and they know you. Don't get ratted too often. NEVER throw up (seriously uncool) or strip (OMG!).
Have a great time.... I did!