From when I was younger, like 12, my rule was 1 year younger, 2 years older.
There was a girl that I really liked, who was a little more than 1.9 years younger than me, and I sometimes felt bad liking her, but, well, I did anyway.
Nowadays, I'm 19, so I say no more than 2 years younger. Being a person who felt that 20 was old all my life, it might be kind of hard for me to accept someone over 20 not being really old.
Just a little quirk of mine.