According to The News International:
Prince George will be gearing up for an unbending royal rule in a year’s time as he turned 11 on Monday.
The young prince, who is the second in line to the throne, will not be able to travel with his father Prince William…
…royal author, Christopher Andersen [said] “Starting at age 12, heirs to the throne are required to take air trips separately to preserve the line of succession should an accident occur.”
“It’s a morbid rule, but then again, when they turn 16, all senior royals are also asked to help plan their own funerals,” Anderson said.
Source: The News International