Why do younger girls go for me the most?

I’m in my mid 20s and every time I go to the gym, I’ve been getting hit on by high school girls. I’ve been asked out twice already this month alone. But I don’t like younger girls, and I already have a girlfriend. There’s even this one girl who follows me everywhere and stares at me consistently. I’m flattered, but I wish it was girls my age or older. I don’t tell my girlfriend too because I don’t want her to worry.
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It’s also weird sometimes, because I’ll literally have girls fangirling as I’m working out, and I try to ignore it.
Why do younger girls go for me the most?
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