As I work very closely with SEO/SEM, would it surprise you that the average age of people on Facebook is actually mid-30's to mid 40's? It's true! (I'm 41 BTW)... many of the 30-somethings and 40-somethings have used FB for professional uses (such as myself) but also use it to reconnect with old friends from high school and college as well as family members across the world. (Studies have been shown on this!)
FOr example, I've recently put up a fanpage for my business on Facebook...granted, it's only been up for 2+ weeks, but I have over 100 fans already and the average age (according to the stats on the fanpage) falls in the 35-44 age range. It's not just for the 10-25 year olds anymore!