The easiest example is, what age should someone be allowed to transition which is an intensely challenging question to answer even on a medical level.

That actually has a really simple answer, the right age is the one that the person and their doctors/medical professionals consider age appropriate for that individual. It isn’t up to society to restrict that decision. That is before the fact that medical professionals with direct experience with the person will have the best opinions on the topic.

This is also true for every single medical decision. Also true for every decision that doesn’t directly harm someone else.