I am 38 years old myself, and a regular Facebook user. I have roughly 40-50 "friends" on there who are also regular users, all over age 25 -- and all are people I really did/do know in real life.
Most of us use it to share interesting web URLs to stories we know our friends would like to read, and to feel like we have some idea of what the rest of us are doing these days. (One of my friends moved from the USA to Taiwan, and another moved to Spain - so the time-zone differences alone make it impractical to just call on the phone and have a conversation.) With Facebook, you not only feel like you're "keeping up" with them, but some of the photos they share add a lot of value too. (My friend in Spain used to talk about the beautiful sunsets there, which meant little to me until she starting posting photos of some of them. Absolutely amazing stuff!)