I was a late bloomer and definitely sheltered, so the phrase, "BJ" didn't appear on my radar until I was a junior in high school, and then it was only in reference to my cousin talking about a girl I went to elementary school with. He said, "they say she's good at giving bj's."
I, of course, had to know what a bj was, so I then hounded my older, male cousin to explain it to me, and he awkwardly said something about a girl putting her mouth on a boy's "thing"
How about you? How old were you and in what context did you learn about them?