A good basic rule I've heard of is "half the age of the oldest partner + 7 years" to be the minimum age of the younger partner. This is kinda a sliding scale since it means that the older you get, the bigger the "acceptable" age gap.

I think it's a good idea in principle, but I did go out with a 35 year old for a while last year, when I was 20. Didn't really work out, but I'm not entirely sure if the age gap was to blame. More me still being in love with someone else (my current partner, who is a month older than me).