People can do whatever they want to do. Just because someone is under 18 doesn't mean they are as immature, dimwitted, stupid, etc., as people think. Albeit a vast majority of them are. If someone would rather date a mature male/female from a different age group, then let them? I don't see why someone would have to date their own age if their own age group is filled with idiots. Love and relationships have no guidelines, there is no rule that states you can't have emotions for someone because they are 10 years younger than you. Perhaps if the law wasn't engraved in people's skulls you'd have a different perspective over the whole matter, but sadly you can't say that it wouldn't change, and I can't say it would.
People need to be more open minded about it. Now you may ask if I would let my fictional 16 year old daughter date a 25 year old man. My answer would be no. Why? Because this is from a totally different perspective, as a father, of course I wouldn't want that. Hell, I wouldn't even want her dating an EIGHTEEN year old boy.
Whether they are being stupid by dating someone outside of their age group or not also depends on the values and examples given to them from their guardians. If they are smart enough to have feelings for someone who actually cares about them, then what exactly is wrong with the relationship other than this apparent taboo age difference? Nothing.