I would say the same age as children identifying as animals, like a doggie or kitty. At the age of 2-4 children like to pretend they are a different species; other than with human DNA. They often copy the behavior and sounds of the household pets. That is when you get them, just before they outgrow it. Nah!
One second thought, it may be better to let them make a few semi-adult decisions, like driving the car, so they show some adult responsibility in potentially dangerous situations. I guess 18 sounds good, since that is the legal age of sexual consent, when you are considered adult enough to do most things, except legally drink and smoke. That has to wait until 21.