There have been quite a few threads about people being moved because the FA's or pilot or whatever did not think they qualified to be there.

Not sure how you can make a determination as to his worthiness to be there based on him taking his time through security, but that's another story.

No, I don't believe they can reseat somebody based just on age, nor should they be able to.

Edited to add - There is a minimum age you have to be in able to seat in an exit row. While this thread is clearly not about anybody near the minimum age, I should have spelled that out to avoid the confusion apparently it caused.