Massa was flirting with Wasserman.
Why is this a red flag? Because this was a interview with a journalist, it is inappropriate to try to come-on to your interviewer. If Massa would flirt with the young reporter, he apparently doesn't understand appropriate social boundaries in the workplace; that's the "red flag".
I'm rather reading between the lines. There are other reasons why someone might ask for the age. One could be implying that the interviewer was too young and naive. But to repeatedly ask for the age is inappropriate, whatever the reason.
In the book stating (I paraphrase) «Massa (who later resigned due to harassment of a young male staffer) repeatedly asked me (a young male reporter) my age» is, I think, a pretty clear implication that that is how the reporter interpreted the situation. Even if it wasn't flirting, The age of the interviewer has no relevance in this situation, so asking about it was inappropriate, whether it was actually flirting or not.