Birthday Swap

9-yr-old boy. 

Boy: Oh... That concert isn't on my birthday is it?
Me: When's your birthday?  It's in May, right?
Boy: Yep.  May the 24th. 
Me: And you're turning... 9?
Boy: 10. 
Me: Well, that explains why you're so tall.
Boy: Oh ... Wait... My dad and I are swapping.
Me: You're swapping?
Boy [starting to giggle]: Yep.  He's going to be turning 10, I'm going to be turning 39.
Me: 39?!
Boy: Yep, I'll be 39.  So... Actually... My birthday is now going to be April... No... March 30.
Me: Wow.  So you'll be the oldest person in your class!
Boy [big grin]: Yep!