There are a lot of rules in the royal family - but there's one major regulation that kicks in when a future monarch turns 12. Because Prince William is next in line for the throne and his eldest son, Prince George, is about to hit a milestone year (he'll be 12 on Tuesday, July 22), they can no longer travel together. Various royal experts have confirmed that this is less of an official rule within the royal family and more of a traveling protocol that's been in place for years. Essentially, it protects the two future monarchs in case something happens while they are traveling.