Depends on how old you are. I'd say that as long as you're both at least 18 it's fine. Even though most people would freak out then, so over 21 is probably better socially.
I think that at 3 years difference if you aren't doing anything more than just kind of hanging out and chatting or watching movies, its ok wether you are both over 18 or not. Its when it gets past that that it becomes wrong.
Well if you REALLY love her well then date her, And if it's just for kicks get somebody else then. Dating somebody way too young for you isn't the best way to go unless you really like a person.