People change a lot after being in a major new phase of their life for a year.

What is this major new phase you’re talking about? graduating high school? I fail to see how that’s related to relationships. Also some people graduate earlier or later.

But a 20 year old meeting a 17 year old is significantly more creepy

Is an 18 year old meeting a 17 year old also creepy? Where do you put the limit? Why is a 30 year old meeting an 18 year old suddenly ok?