Well first thing's first, let's get one thing out of the way: children say the darnedest things all the time. Some of the shit kids say, especially at a young age, while baffling at times should rarely be taken as seriously as an adult. This is because their cognitive abilities are only just starting develop; which is why they're really impressionable. I've met kids who were totally convinced that they're actually cats, but then outgrow said phase a few weeks later.
Ask these people why you have to be 21 years old in order to legally be allowed to drink in most states. Why do you have to wait until you are 16 to be able to get your driver's licence? Why is it that the age of consent in most places is 16? Why can't kids legally purchase firearms until they're 18?
These are important questions with obvious answers: because we cannot legally trust children to be responsible.
This is why in most places juvenile offenders get lighter sentences than adults do.
So why should we be able to trust that they really have gender dysphoria at a young age? I personally believe that gender dysphoria should be treated in the same manner that most mental illnesses are in that it cannot be diagnosed until the patient is in their 20s and has actually met the criteria for it.