Look, you just decided that since your characters are uglier, the maturity level is higher. Well, I have news for you! Since the expansion, every "let's run around naked on female characters" pervert is now a blood elf. They're horde. Lol.

I have also played both sides, and have yet to notice this magical difference. The amazing thing is the majority of players have characters on both the alliance and the horde. On pvp servers this is made harder as they have to create the other-side alt on a different server, but it still happens just the same.
So, how is it that these players who play both alliance and horde are suddenly more mature when they are playing their horde characters (due to ugliness, of course), and less mature while playing their alliance characters?

Also, my personal experiences negate this "horde is older" theory. My Alliance guild is made up of many people who are married (and many of whom both the husband and wife play). This includes both my parents, a 70 warlock and a 70 paladin.

Also, barrens chat.