As Prince George celebrates his 12th birthday on Monday, July 22, the young royal will quietly reach a new milestone in royal life — one that has sparked conversation among fans across the world.

The eldest son of Prince William and Princess Kate, George is second in line to the British throne. And with each passing year, he steps closer toward his future role as King — including now having to follow a traditional royal protocol that restricts how he travels.

Why Prince George may no longer fly with his father

From the age of 12, royal protocol suggests that two direct heirs to the throne — such as Prince William and Prince George — should not travel together on the same flight. The custom, while rarely enforced in recent years, is meant to protect the line of succession in the event of a tragic accident.

Prince Charles (now King Charles III) and Prince William both began following this rule around the same age. While official policy on this has sometimes been relaxed — particularly in George’s earlier childhood when he flew with both parents — it is expected that the guidelines will be more closely followed as he approaches his teenage years.