Not to be a jerk, but that isn't an illegal question. Calling people "stupid millenials" makes you sound petty. If this is how behaved in the interview, I have no doubt to why they asked you that question. And you are a millenial, you're 31 years old.
Actual it is illegal -- it's basically asking age, which is illegal.
I simply stated that many times in PM, you tend to face resistance or (even worse) ignorance in obtaining support or sponsorship from upper management. You must use your negotiation and presentation skills to overcome these situations.
Use the 'SAR' (Situation, Action, Result) method to answer this type of questions.