Try not to worry.
Yeah, while the avg MPP student IS older, it isnt by a whole lot (though it does vary from school to school). There will be others your age/situation too, especially given the economy inducing/forcing UG graduates straight into Grad school. What is more important is the school itself. Is it a fulltime cohort-style program or is it more of a commuter school. The former will be easier to form friendships, the latter will be more difficult. My previous experience in grad school was at a commuter-style school and I never formed a single friendship. Of course I never plugged into the activities and only showed up at night at class time. But at least I was familiar with the city and had an unrelated social life. Bottom line, get involved.
To hit other concerns, 1) You will probably have to work with fellow students on group work. (most MPP programs have some classes with a group component) So you can get to know fellow students in your group...offer/suggest going to a bar/cafe/whatever to relax after. 2) Married students arent necessarily unavailable or unwilling to socialize, they just have a more significant significant other. Yes that may make heading out for a late night drink more difficult, but that doesnt mean they'll ignore you either if you dont have a partner. 3) I'm in this category, probably a dozen years older than a graduating senior. So, I'll say some things from personal experience. Yes I have a family and all the obligations that go with it. It will make socialization with my cohort more difficult (this actually concerns me because, I will also be new to the school and city). But it doesnt mean I'm going to ignore someone younger either. It's always good to have a friend ten years older (or in another stage of life). It can help broaden your perspectives. One of the first friendships I had in Chicago was with my immediate supervisor. While he was only my supervisor for several months, we remained friends for more than a decade since (was in my wedding and when I took my family to Chicago for vacation, he offered to let us stay at his house). So sure, an older friend probably isnt going to hit the clubs with you on the spur of the moment thing on a thursday evening. I know I wont. But I'll hang out after class when I can. I'll respect you as an adult, just as I do with my cousins who are all in their early 20s. I'd even host a group study session, if time and space permit.
So hopefully you can relax a bit.
Good luck