Haha, I used to be just like you when I was at school. 17 and 20 isn't such a big difference,
I honestly have no problem with it if they were friends before hand for a while and then started dating. But when it is something like a guy just doing something in his everyday life and sees a girl (sorry bogey but I saw your picture and you don't pass for 20/21) that he KNEW was not his age and tries to get a date with her- that annoys me.
I also don't believe the word "drinking" should come up until you've been dating a while.