It depends on the local laws of the origin and destination whether carrying vapes on a plane under 21 is illegal.
Possession of disposable vape is allowed on a plane if you take it with you or put it in your carry-on baggage. However, purchasing them when under the age of 21 can be illegal in some places.
In some areas, if a person under 21 years old is found in possession of a vape, authorities will suspect them of illegally purchasing it. As a result, we do not recommend bringing vape on a plane for those under 21.