Hey at all!

I've got a question:

When Bella gets her high school diploma, she's 19 (there are just a few months until her 19th brithday, but it's in the same year). So, my question is, why is she that "old" when she gets the high school diploma?

I'm from Germany, so I don't undestand the american school system that good. Could you explain this?

Thank for answers and sorry, if my english is not that great.

fip