Most Helpful Girl
Just tell her the truth. Tell her maybe when both of you are adults you can talk about it again. Tell her it's kind of pedophilic for an 18 year old dude to be dating a 13 year old girl. Besides every young girl has a crush on some older dude at some point. It's life.
Most Helpful Guy
"I don't know how to let her go ithout hurting her"
we're talking about a 13yo here, the only way to make her get the point is by hurting her. Yeah, call me evil and all that, but seriously, girls in their early teens with a crush can be hell (And blind as bats too).
Tell her EVERY single reason why it can't work. Your goal will be to make sure she drops you, and any fantasy of older guys. Tell her why it's illegal, why it's morrally wrong, why... basically every reason there is for her to not do it.
She won't like it, and will get pissed, but come on, you got litterally nothing to loose. Shoot her down, and make her give up!
Brutal? sometimes life is brutal, and sometimes what you need to hear is not what you want to hear, and what' the right thing to do is not what you want to do. And don't forget, the road to hell is paved with good intentions. Don't walk her to hell by sugar coating the truth please