Earlier this year, we had the shocking news that the fastest growing group of Facebook users were the 55-plus age group. In a space of just six months, they had increased a whopping 513%. Time Magazine's Lev Grossman came up with reasons why Facebook was for old farts: Facebook is for finding people you've lost track of, by the time you're in your 50s, you're no longer bitter and twisted about high school (if you can remember any of it), you're at an age where you don't get smashed at parties and get photographed in compromising positions (even if you wish it did happen), it's a business network and you don't understand Twitter.

At the same time, Twitter isn't exactly for teens either. As reported here, a research note written by a 15-year-old Morgan Stanley intern says kids just aren't interested in Twitter (it's easier to just text).

New research by the ReadWriteWeb site shows that Facebook is getting older and Twitter is the second youngest of all the social networking sites with strong uptake in the 18-24 and 25-34 age groups. According to that research, Twitter's median age is 31, MySpace's is 26, LinkedIn is 39, and Facebook is 33.

But that's the figures for now. Next year, it's likely to be very different. Twitter and LinkedIn will get older as more old fogies take to it.