I look younger than my age and my boyfriend is 4 years younger than me. I don't see the problem(s) (yet). But 10 years older is the maximum and if he's younger than 5 years maximum
I have once been told before that the rule is "half your page plus 7". Of course, its not a finite rule, but something to consider.
Any age is acceptable as long as you love and care about each other and are able to connect at all levels.
Usually my acceptable age is usually 10 years older but if he's older than me and he loves me vice versa and we care about each other it doesn't matter about the age difference