My rule used to be that they had to be within 25% of my age for me to go out with them, but then I met Danielle and broke that rule 'cause I couldn't figure out any other reason not to go out with her. So, with that being said, noting I'm 22...
Too old? I'm not into older women in general. 24 is probably pushing it with me.
Too young? So long as they're legal and it doesn't weird you out then I guess it's up to you. But, uh, try not to get too far out of it. The youngest a girl could be for me to go out with her is 16 years old - Danielle's age and the legal age for consent in the UK & NZ and therefore in my head, it's also the legal age.
When I'm 30, I wouldn't hook up with a 16 year old though. It'd probably be more like 23-33 at that point. Hopefully I won't have to think too hard on it though.