I've done the higher/lower thing.
HB: How old are you?
Rhody: Guess
HB: 27
Rhody: Higher
HB: 28
Rhody: Alright, if you're going to go up in increments of one, we'll be here all night. I'm 39
HB: No way, you're lying. Let me see your license
Rhody: I'll show you mine if you show me yours
HB: You don't believe I'm 28?
Rhody: I don't believe your name is Brandi
Being older, I have the opposite problem. I don't run into many cougars, because what most of you consider a cougar is a woman my age. So with younger girls, being playful usually diffuses the age issue for me.
Maybe being more mature and serious could diffuse the problem for younger guys dealing with cougars. For younger guys, I would tell her to guess. If she guesses low then say, "are you saying I look immature? Actually, I'm [your age]." If she guesses high then say, "wow, thank you! Actually, I'm [your age]."