After a year together, the gap doesn't seem as bad because we all have to do the same work, you know? We all go through the same methodology sequence, so bonding over problem sets happens.
But I can't say that I'm really "friends" in the hanging-out sense with anyone in my cohort, the way the younger students are. I'm in my middle-aged world, where I commute 1/2 hr to campus, and they live closer by. I do feel a little left out, but that's OK. I think that if I were also TAing, we'd have more points of connection, but I'm self-funded. (Working part time at my civil service job pays 2x what a GA would.)
Going to happy hour as a group can help, too!