He laughs, because he thinks she's either making a joke about their respective ages (she is, but not in the way he's thinking) or making a joke about how as a cardinal who's devoted his life to the Church he's necessarily a bit sheltered.

I mean, why not both? The answer is probably both.

"If you're trying to do the equivalent of a bartender asking someone clearly old enough to drink for their ID, I'll shamelessly take the compliment."