In the British royal line of succession, the heir apparent is the eldest child of the monarch; currently, Queen Elizabeth II's heir apparent is her eldest son, Charles, Prince of Wales. Next in line to the throne is the heir apparent's eldest child -- Prince William, Duke of Cambridge. When a direct heir has children, those children follow the heir in the line of succession, preceding the siblings of the direct heirs. Thus, Prince William's children precede his brother, Prince Harry, in the royal line of succession. According to British royal protocol, the heir to the throne and his eldest child may not travel on the same plane, to protect the royal lineage in case of a crash.
The British Royal Family almost always follows this travel protocol. However, there have been several occasions when the protocol was broken. For example, Queen Elizabeth II gave special permission for Prince Charles, Prince William, and Prince Harry to travel on the same plane after the death of the Queen Mother in 2002.
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So would protocol prevent Prince William from flying with Prince George? Since they have flown together in the past, is there an age when this takes effect?