I didn't until I was fourteen. This was pretty much the story:

Me: Mom, I have something to tell you.
Mom: What?
Me: I'm scared to tell you, though...
Mom: You don't have to be afraid to tell me anything, son.
Me: Mom I'm... *five second pause* gay.
Mom: Duh.
Me: <3 <3 <3

My parents ALWAYS figured I was gay so I don't know why I waited so long to tell her. I must say that I'm blessed to have such an accepting and loving family who never really cared either way, and it's unfortunate that more families aren't like that.