I think your distinction between teen coming out and 20's coming out is fair. I mean, the teens are a time for trying on new personalities and sometimes that could mean questioning their sexuality (actually, girls usually do that in college and I'm not sure that guys do it at all. Its different for girls...more socially acceptable).

Gill and I have had several talks on the subject and I've basically explained what being gay is (her 'uncle" Erik, my best friend, is gay so that helps) and I've always told her that, when she is old enough, she will know which gender she's attracted to and whichever it is, I will always love and approve of her. To which she replies, "It's guys, mom."
She's 11 so I'll take her word on it for now.