Among the East-ish dorms at MIT, a certain brand of alumnus is called crust or cruft. Having gone to MIT merely makes one an alum; the truly crufty are those who hang out mostly with other alums and current students, maintain connections to their undergrad dorm or living group, and participate in undergrad activities (gaming, LARPing, and hacking are some of the popular ones). In some ways, being crusty means that you remain a cool person after graduating instead of selling out and turning into a boring grownup. But carried to its logical extreme, cruftiness becomes a sad attempt to hold on to something that is no longer yours; it becomes an inability to grow up.
For the most part, I think that having older friends around is a good thing for undergrads (most crufty folk that I know are of the non-extreme variety, with lives not solely defined by MIT any more). They remember the last time somebody tried that Bad Idea. They can remind you that actually, that "ancient tradition" was started six years ago, with their help. They can both tell exaggerated "Back In The Day" stories and set your facts straight about the ones they heard when they were frosh. Just watch out for old drama if you make lots of crufty friends.
"Crusty!" also makes a useful joking-insult for someone older than you (graduated or not).