Ok...so my little brother is a bad source of information that changes from one day to the next and I apologize for any confusion, but I think I finally nailed him down on the exact wording and I think it may make a difference.
"Mr. Johnson is 31. This year will be his 34th Christmas. His niece is 30 and has had 29 Christmases. How is this possible?"
It is not much different, I know...which is why he thought the wording was not a big deal. Anyone with much experience with riddles, though, knows that wording is everything. If 31 and 30 do not, in fact, refer to their ages, this could be something entirely different.