My 15yo son just joined a Fantasy Football league with his friends. First year he's done that. Previous years he did it with me. I'm sure he'll still do one with his old man if I asked him to, but his heart will be in the one with his friends.
I'm not gonna lie -- I've been a little emotional today, realizing he's starting to (naturally) drift away. After my wife, he's my best friend in the world.