There is no set age limit for when Prince George, or any other member of the royal family, is not allowed to fly in a plane with their parents. However, as a general practice, young children are often discouraged from flying in small private planes due to safety concerns.
Prince William, who is a trained pilot and has worked as an air ambulance pilot in the past, has been known to fly small planes as a hobby. It is possible that he may choose not to fly with his children, including Prince George, in small planes as a precautionary measure. However, this would be a personal decision rather than an official rule or guideline.
When traveling on official royal engagements, the members of the royal family usually travel on large commercial planes or chartered flights, which are subject to strict safety regulations and protocols. In such cases, Prince George and other young members of the family would be allowed to travel with their parents or other family members.