I have struggled with this problem my whole life. I have always seemed to make better friends with people older than me. I thought it would get better as I got older. It did a little bit, but very little.
I think that this has to do with a lack of common interests and experiences. While a person is focusing on sports, social activities, etc, we our focusing on our special interests. Friendship I have found, is made on common experiences, personalities and interests. People who are older (or in some cases younger) may of had more of an experience with that special interest than most people our age.
What I am running in to right now is that people who are my age are focused on their careers, families, tv shows, and drinking. Since I am not married, I don't have kids or a spouse to talk about. I have a full time job, but it is not the center of my universe. I very rarely watch tv. I also very rarely drink, and even more rarely drink with other people. The only places that I have seemed to make any friends my own age is when were involved in the same experience or interest. Some examples include the basketball team that I was on in high school, my church and my volunteer activities.