Respect v. Maturity
I would have been in that position and I absolutely, positively, would have had no problem with it. It's all relative. I figure that if I had started fifteen or twenty years before we'd be about the same age.
I figure that this other person is there for a reason. He/she is the Captain, age difference notwithstanding. I would have had no problem with it whatsoever. Even if the captain was young enough to be my son or daughter or, a couple of years earlier, was my flight student (and, in effect, I was the Captain).
It all turns on respect. As PuffDriver mentions, maybe the problem is more with the FO in question. If, indeed, the FO is some former military bigshot who got into airline flying later, that person needs to swallow some pride. He/she needs to realize that he/she is starting out again and exhibit humility. On the other hand, you do get some 23-year-old captains who are not especially mature by anyone's standards, and need to learn respect. Maybe they can learn from the situation as well.
PS-This is a good discussion. Also, a potential interview question.