I don't know, but when I was in 8th grade or whatever, I often felt like I could approach ANYONE for at least a little sympathy. People were willing to talk to you if you wanted to talk to them. It's only later on when people have more experience with social interaction, and ultimately decide that it's "better" for them to simply shut out certain types of people. Especially if they had bad experiences with friendships/relationships - they're always less friendly the next time around.
Maybe they get frustrated with certain types of people. Or whatever. It's so HARD to find smart people who are willing to talk to you for any length of time past the age of 16. Especially girls. At least many boys are still very immature well into their 20s, so they're often more willing to talk.
Of course, I'm no exception either, since I'm hyper-stressed over my schoolwork.
And marriage, in particular, is one of the worst effects of age. Studies by Robin Dunbar et al. have shown that people who marry lose an average of 1-2 close friends. Sure I totally understand why they do it - but it certainly does make them harder to approach. Especially if you aren't lawful to begin with (and my severe ADHD often forces me to pursue non-lawful routes.
And it's also quite easy to see that older generations (and especially married people) are more likely than others to disapprove of victimless things like gay marriage and marijuana. While there may be some legit reasons to disapprove of them, the primary reasons seem to be related to intolerance. Of course, part of it is simply a generational effect, but part of it is also that they want to control their children.