Personally I think it is more about mental than physical age (still assuming that everyone included is consenting adult and all that...) And yet somehow I've gotten this idea in my head, that 10 years is fine, but if you're dating someone close to your parents/children in age, that there must be something wrong. ._.
Me? always been in to older men myself (+10 or so) and could never think being with man who's not at least few years older than me. And luckily I found man to suit my age difference preferance. With women I would have been ok with +/-5 years... strange, when I start to think about it. I shouldn't.