I joined a grad scheme at 26 (one of the bigger strat consultancies, not Big 4) and found my British co-workers (though not other Europeans), mostly 22-23, a little immature. Not a huge problem. More annoyed with myself for being so late to start, which meant I was concerned about promotions etc., so if you're the sort of person who compares him(her?)self to peers might get stressful. Seems to me most consultants are this sort of person. Being mature helps in front of clients, if you're joining a firm that will actually expose you (cue jokes) to clients.