They have 5 year olds declaring that they're transgender, and 26 year olds who can't be trusted to get their own health insurance. Every number is subjective. The law says 18 years old, so United is trusting 16 and 17 year olds to behave themselves (They don't have to.. ). Obviously they are losing customers to Airlines who's policy goes younger, but I bet there is an economic reason to not matching the age. (probably insurance related).
Now, we can discuss whether the price for this "supervision" is reasonable. How much does a babysitter cost?