Like, minors are notoriously not great at making decisions or being reasonable, I wouldn't trust myself doing something this big, that's for sure.

But a minor isn't making this decision alone. At the very least, there are all of the medical professionals who have a checklist of things to work through before recommending and clearing any particular procedures. Also, its going to be pretty difficult for most minors to get any kind of gender affirming medical care without a parent being involved. So before there's any arbitrary limits to care based on age there's going to be a small army of people who are already directly involved with the decision making process alongside the patient.

Removing the arbitrary age limits allows for patients who are developing faster than the average that the age limits where based on and their doctors to move forward with treatment based on the individual instead of being forced to wait until the correct age has been reached for the doctors/ect to begin treatment.