My rule was always you have to be legal to do te same things (i.e....if you are over 18 and can vote, now one under 18, can drink? no 18 year olds) and neither can be capable (years wise) of being the other one's parent (once you start getting into the teens difference wise, it's a no go)
Of course I broke this rule briefly, twice, and I can tell you I shouldn't have!