99% of the this kind of quasi-resistance can be avoided if one simply ignores it, which sounds ridiculous on paper but is surprisingly effective in practice.
Instead of letting the conversation die, simply insinuate what you already know (that she is attracted to you despite your age) - "If by 'too young,' you mean too virile and sensitive to a woman's needs, then I agree." - and then continue as if nothing was said at all.
Alternatively, you could always quote the old techno song, "Age ain't nothin' but a numba," but then, you might be too young to remember that
Your boy,
870