How come it feels icky to talk to an 18 year old as a woman but for a lot of men they find it fine?

I was talking to this guy online and everything was going great until he said he was 18 and still in high school.

I instantly got the ick from that but a lot of men my age or higher would go for it just because it's technically legal.
How come it feels icky to talk to an 18 year old as a woman but for a lot of men they find it fine?
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