Tell her she’s coming on wayyy too strong

shes trying to trap you into a world of bankruptcy, insomnia, and slavery.

The twist in the tale...his girlfriend is 31!

Teens want lots of things. They’re irrational and hormonal.

You tell her kindly that it’s lovely she feels that way and you’re flattered. That you could see yourself marrying in your late twenties and having kinds in your 30s if by then you’ve found the right person (or whatever it is you’d like your life to turn like), and that you’re not against the idea that she might en up being that person, but that so far you’re just enjoying getting to know her and that you like her (if it’s true). Also, that that kind of thought is far from crossing the mind of most young males, but that you understand that you’re wired differently and hope she doesn’t make it a big deal. She’s just unexperienced and upfront. Probably the bubbly « no filter » type. Nothing wrong with that in my book ;) Enjoy young love

I'm going to suggest not panicking. Teenagers are known to express affection by saying things they've heard older people say. She may have intended no more than "I think you're a keeper," and simply phrased it insanely. However, just in case ... don't have sex with her.

Run as far away as possible. Or make her think you have amnesia and dont remember her

Is your girlfriend 48 or something?