If he's asking if he's too old for you, he already knows the answer. If you were interested in him you would already be flirting.

Taking his shot is one thing, but hitting on someone who you know has no interest is just rude. As you point out, it puts them in a social bind. It's him trying to force you to engage in flirtation in the off chance he can impress you somehow.

You owe him nothing. Do whatever it takes to make your lack of interest clear and move out of his physical proximity. He shouldn't have asked you that question in the first place.