OP I mean this in the kindest way possible. But please do this young girl a favour and eliminate all contact.
1) I was almost that 15 y/o girl. As a 15 y/o then, It feels so fucking amazing and flattering that a 20 year old guy has feelings for me and is chasing me. That is not a mature thinking. You claim she is mature for her age, a girl who is mature for her age, would not be flirting with you (no offense or maybe we both have different definitions of maturity).
2) if you pursue this, she may have a wakening moment and resent you. Feel as if you robbed her of her innocence, youth, etc. Because I resent my predator.
3) you’re the adult here, you have the power and control and let’s say you pursue this relationship, your relationship will always have some sort of power dynamic. She may always look to you as the authority figure because you’re so much older
4) she is 15, and you are 20. There are parts of her brain that are not fully developed. There are life problems where you won’t understand each other on so many levels. For example, she may have a peer conflict and that may feel like her world is ending vs your conflict would be trying/struggling to meet rent where your world will feel like it’s ending.
5) Being 15 she is so young and doesn’t need to be in love or pursue a relationship with a 20 year old. She has so much time to do the dating field. She needs to focus on school, learning to socialize with peers her age group etc, learn what hobbies she likes and does not like. Same goes for you! You’re 20 and yes you are young too, but you have a much bigger world and dating pool. You will meet so many other clever, intelligent and mature women of your age!
6) ever thought what her parents and peers would think? Imagine having to meet your 15 year old girlfriends parents… I would not want to be in your position. The only reason why I did not pursue my relationship with my predator was because my friends threatened to call the cops. They did me a huge service and gave me a huge reality check. - don’t risk your own future as well
7) if you’re still feeling super attracted to this relationship, wait 3 years.