Most of my friends and I met and married our husbands when we were in our early to mid- 30's. There were a couple of my friends who had finished with their college graduation "starter marriages" and remarried in their 30's and have gone on to be much happier than they'd been before. There were a few of us too that had been in longer term relationships that didn't work out. I don't think it was all one way or another.
I would say that for most of us, the age we married at had less to do with the guys we met and more to do with where we were in life in general. I mean, we all pretty much always dated and would certainly have loved to have found Mr. Right--but we weren't exactly panting for a ring. We were figuring out where we wanted to live, what we wanted to do, going to graduate school and getting the certifications and licenses to make all that happen. We were pretty happy with our lives and getting ourselves established and it seems to me that once we managed that for the most part, it wasn't too long before we all started meeting the guys that we ended up marrying.
I would guess that if most of your friends are in serious relationships, that's most likely because you were in one yourself until recently. It's probably who you gravitated to at that time. And if you're mostly meeting the single guys that are just up for "fun", then I'm wondering where and how you're meeting them? That's half the battle there. I'd be stalking all your friends for setups with their single brothers, buddies, cubicle farm mates, etc. that they think might have something in common with you. Or looking for social groups or volunteering opportunities where common interests might come into play instead of places where the expectation may lean more toward "hook-ups".
Either that or you might take a look at your dating finesse. I'm not suggesting anything about you personally, just throwing an idea out there--but sometimes playing the "player card" can be an instant version of not calling. Making themselves unattractive and taking themselves out of the pool, as it were...
I've always had a theory too that there are a couple of stages in life where people tend to get married en masse. Early on--right out of school and then it seems like you see another matrimonial "push" in the early - mid 30's. Either 2nd marriages or more commonly to me, marriages once people have established themselves a bit more. I'm still finding out how successful those are though