European tours for young people?

Any advice or recommendations for tour agents or packages to Europe that cater to a young (twenties) clientele?

I would like to offer my son a trip after he graduates, but I’d prefer to find him a group tour that is somewhat structured and orderly, with a guide, and not just turn him loose. Can you recommend any reputable outfits that offer packages geared to young people? Me, I wouldn’t mind traveling with a group where no one’s under forty and wandering around landmarks with a bunch of retirees, but it might not be as much fun when you’re 26. (Actually I did exactly that when I was 27, but I don’t expect him to find it as easy as I did.)