My girlfriend likes to pretend she's 13 when we're in bed together, which I told her is weird and pointless since she will be 13 next year.
I don't get it, is this a joke?
No, what I mean is when she tries to be a child.
It sounds to me like you're making a pedophilia joke, which isn't okay. If she's pretending to be a child that's sort of gross. If she's really 12 going on 13, that's disgusting and you ought to get some help.