I think if two adults (or two not-yet-adults) want to get it on, it's up to them to work out why they like each other and what they want from each other, and to both be responsible for what's happening, regardless of age difference. But in practice, if someone a lot younger than me expresses an interest (and it has happened once or twice, and is very flattering!) I'm suspicious about what they're looking for - and with good reason, because the two people I've dated who were 10 years younger than me both ended up casting me in the role of their dad that they were rebelling against. No thanks.
(Note for the polyamorous among us - this is a deliberate oversimplification!)