Well we are going to the movies tomorrow, I think tonight I might just say to my parents that im going to the movies with *her-name* (and explain that I met her on the bus etc).
It might be best to just play along and not mention her age at all and get away with it. She looks much younger so I think that may decrease the chance of me having to explain how old she really is.
I could imagine that one day my parents would like to meet her. I mean my parents can't stop me, All they'd want is for me to be happy, and I am, also I am nearly an adult, so I can do what I want.