I'm working in a similar situation. Bigger outfit, and I'm a bit older, but unless the culture is really off the rails then I believe it's all about how you handle it.
In my case, the key is to not exclude myself. I definitely don't participate in all the extracurricular activities, but I do join in enough to stay part of the scene. Yep, I've been to one of those epic Tahoe long weekends. Disc golf at the park. Drunk at the office. Barhopping in the Mission. But only once in a while.
Even the occasional late-nighter is doable; in my case the girlfriend travels for work occasionally, so I just load up on coffee and Dew and code-rage with the gang when she's away. Your situation will be different, but I bet there's a way you can crunch hard a few nights a year.
I've found that if I go out of my way to fit in, others go out of their way to include me. It helps that I'm "youthful" (a nice way to put it) and active for my age, and have hipster-friendly interests since before they were cool, like rock climbing, cycling, and whisky. But I only participate in a fraction of the party mentality and no one seems to mind.
So I'd suggest jumping in! Just be yourself and don't let the grown-up pants get too tight. Focus what you can do, not what you can't. Hang out late once in a while, teach the young bucks how to hold their liquor, go on one of those Tahoe trips. Chill with the crew on a Saturday afternoon, then bow out when it's time to hit the clubs. Just keep it at the level that works for you, stay positive, and have fun.
I've even been able to bend the culture where I work a bit. More stuff is SO-friendly. A few peoople have quietly aspired to more "balance." And some days the chess set gets more action than the (obligatory) foosball table. It's okay to be the old guy. Own it. Make the place better for it.