Not that I'm a PhD student or anything, but from my perspective you need to rephrase your problem a little, and yes it may be possible but it's dependent on what kind of person you are.

I only decided to go to Uni at 26. There is a big difference between me and those of my classmates who are 19, but there are enough of them nearer my age. I don't go out with them all the time, but I do have the odd evening out with the 19 year olds and it's fun and enjoyable. I also go out with the people nearer my age, and my boyfriend "who's older" and those places can vary from small intimate local music nights to old-man pubs. I know though that I'm thoroughly enjoying this experience as I always felt I missed out on it when I was 18. But then again, I'm very aware that this experiences is very different to the one I would have had at 18.

What I think you may be saying (correct me if I'm wrong) is you'd like more of a social life, rather than the traditional undergrad experience of getting so drunk you throw up over yourself, wake up in someone else's bed, missing lectures because you're hungover, being too lazy to read the set course books etc.. (yeah, look how high I hold other undergrads!)

What I'm trying to say, but being rather long-winded about, is that the problem isn't weather you're too old to 'fit in' with the Undergrads (because they're such a varied bunch that doesn't really make any sense). You can do that. I am a member of a society and have found it incredibly welcoming and enjoyable and good for my social life. But I can only manage one society, any more and I'll be too busy to actually study.

I know my uni has a lot of PhD socials, where they meet up and chat about whatnot. Can you not look into whether your Uni provides the same? Or any such grad societies or groups? I'd very much advise joining one society. Don't assume there won't be other people there of varied ages, or that they'll welcome you and you'll have a good time.

But yeah, your experience wont be that of an 18 year old and there's nothing wrong with that. You won't be able to socialise as much because you'll be about 99% more busy then they will be. But you can at least improve your position to one that pleases you more.