Honestly, the first time I saw one was when I was like around 9 years old. It was unintentional, and it was together with my older sister. We're like browsing and trying what we got from a flea market of random VCDs (the one before the DVDs) and we watched an episode of La Blue Girl (Injuu Gakuen). I don't know about my sister, but I was still innocent at that time that I just watched it thinking it's another action-comedy stuff with a little sexy moments like Ranma 1/2 and never gave the pussies and boobs being exposed much thought. You know sometimes when the family watches a movie at home and there's a kissing scene? That's how I thought it was when I was watching it way back then.
I only recalled that memory years after when I saw La Blue Girl again.