What would you say should be accomplished socially by 18?

I've been out of the loop for, well, most of my life, I've had just a few short lived relationships but I've felt really entirely out of touch for the last two years. The people I hang out with mostly are younger than me (average 2 years), I wouldn't really call them friends, because there's definitely a maturity difference, but they're my group I guess if I have one. Discluding them I guess I'm just a loner, not necessarily socially behind? My one true friend I play guitar with is almost exactly 2 years younger than me, but he's also like me not quite fitting in with our group cuz he's mature. I'm at his house most of the time we just jam and talk about life, just a couple of bum philosophers. And by mature I don't mean we don't like to act stupid and party, we get our share in. But I just feel so lost and inadequate sometimes. I feel like I've really truly wasted 4 whole years of my youth which are years I can't get back and I'm not sure where I stand. I'm about to turn 18 at the end of the summer and it doesn't feel right, what should a guy my age be experiencing?