Asking a girl who's younger than me for her number before I leave school to university. ?

I'm currently in my final year of school and there's a girl in grade 9 (she's 14) who has interest in me. The truth of the fact is if she were in our year I would already have asked her out and I wouldn't be typing this message. But she's not and she still has to grow and I believe that she needs more experience. I'm in my final year just turned 18 but knew her when I was 17. She likes to talk about intimate things with me and we got a long extremely well. She randomly brings up sexual topics and always touches me compared to other dudes. I leave soon and she says she wants my number when I leave I'm just wondering if I should? If so what then should I do? It's a big shame because if she were as old as me I would've already asked her on dates.

Asking a girl who's younger than me for her number before I leave school to university. ?
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