Would it be ok if it was her university lecturer, or perhaps if she was in 6th form and it was her form master?
Why ISN'T it OK - apart from the reason I gave for you? An 18/19 year old girl is NOT a child and, when it comes to girls, they are a lot more mature than boys and are capable (in most cases) of making a decision. As long as it is consenting, what is wrong?
When I was at school, the PE teacher had a "relationship" with a fifth year (she was 16) and, as far as I know, they are still married.
MCPWhore says you can't beat a bit of new blood - I say you can't beat a bit of experience!
Sorry, I've just seen your last post wrt rattling cages. You're a first time poster (although maybe not new to Pprune) and have made some "tut-tut isn't it awful" comments with which very few people agree. So, if you feel a particular situation is "out of order", then you should speak to the appropriate people. If you're asking for other people's opinions to the matter, you'll find that most think there's no big deal.
Cheers
Whirls