What is the non creepy teenage flirting age gap?

A while ago, this girl (2 - 2 1/2 years younger than me) started flirting with me. We were teenagers. I dismissed her several times because I wasn’t interested but her flirting started to wear me down and I started to cave a bit (we were also very good friends, which contributed a lot). That was when I was 17. When I turned 18 I realized just how weird it was and cut all romantic ties. So my question is, was I being creepy by slightly reciprocating the flirting?

What is the non creepy teenage flirting age gap?
What is the non creepy teenage flirting age gap?
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