I think the irony is that I am quite certain these so-called "immature" people are quite mature, and the reason I am unable to make friends with people my age is because I am nowhere near as mature as them.
A lot of this, I think, would be based on how we define "maturity". In the simplest of models, we still can attribute the source of why one is smart or mature: they can have the so-called "book smarts", where they know a lot of academic knowledge, or a lot of "street smarts", or lots of social knowledge.
I'm convinced that maturity is really a question of "street smarts". We mature by being able to talk to a diverse array of people and select the best of what we see and experience to total to a personality -- our personality. Sharing experiences with each other is far more powerful -- and far more convincing than reading something in a book or learning it in college, especially when it comes to meeting and understanding other people; and in the end, doesn't everything boil down to meeting and understanding people?
With that in mind, most of my friends back in North Carolina are two years younger than I am. I repeatedly tell them that it's easier to relate to them, who are still going through their high school "firsts", than with people my age, who have Seen It All.
My history professor remarked, in ironic jest, that by my age we have become jaded at the world around us, and there is a moment of truth in the irony. In a Scrabble game I played in my Scrabble class a few days ago, my co-teacher drew the word "CONDOMS" straight out of the bag and proceeded to show it to us in the hopes of us laughing. Most of the class seemed to be indifferent -- as if they were Too Mature to Care and had Seen It All. But then, I am convinced I have the maturity of a 15- or 16-year-old, but that's a different thread.