When I was single (I got married at 40), I had a lot of friends who were married. It was never a huge stumbling block to socializing with them. But, that said, a lot of my friends are older than me and either don't have kids or have grown children. I stopped having a ton in common with women my own age when they started having kids and I was still very much single. At that juncture, I started hanging out more with people who were a good 10 or more years older than me. I'm not sure there's any point to that story, but I would encourage anybody looking for friends to broaden your parameters (if you have any).

Another thing I'd add: Sometimes, you have to be the cruise director. If I waited for my married with children friends to invite or include me in their outings, I'd never see them. So, more often than not, I'm the one that suggests getting together and then coordinates the plans. I do that with my single and/or childless friends, too. Certainly before I had to move, I was the one pushing everyone to get together for a movie. If you're not already, take the reins on making plans.

Finally, throw a party or two. I'm not big on parties, but if I was having trouble going to the next level with a group of people, I would consider throwing a party of some kind in an effort to break the ice.