this question still hasnt been answered in a way that can apply to all age groups even with almost 100 years of both research into children who , after learning language & the ability to voice to parents their needs say tney arent a boy or girl they are born as & early childhood development studies on all children including preschool & primary school genderless education programs that dont group activities & learning programs with gender stereotypes.
What scientists do know is that childhood development birth thru X number of years after birth, has phases that are age range dependant. That these types of phases are both innate & taught based, that is understanding & comprehending language directions, & mimic behavior based , where behavior & actions are mimic'd based, both as a form of learning.
what scientists also know that around 3 years old to 5 years old age range, when children learn & comprehend language, if there is a gender identity issue , children correct their parents. if the identitybissue is severe enough, causes emotional distress in the child that is very apparent to adults & even adult strangers.
Scientists also know that some children go thru a phase where they may adopt behaviors & interests that are typically opposite gender behaviors/ interests as part of the learning process & that they do " grow out " of that phase.
That even in adulthood , there are phases of life that are age range dependant.
so the best answer of what age is really individual dependant.
certainly a 3-5 year old age group, hrt, blockers & surgeries are not appropriate.
neither is the 6-10 year old age group.
what is approriate is allowing the child explore & learn without interference of negative reenforcement of non conforming gender behavior to whats considered gender appropriate behavior.
However, if a 3-5 year old has been living & going to school in the opposite gender for 5 -7 years without any indication of continued questioning their identity as the opposite gender, then at the normal ages of puberty ( beginning of the sexual maturity for reproductive phase,) then continuing with medical but non surgical intervention to continue living as the opposite gender is apprpriate.
the hardest group to evaluate whats appropriate course of intervention in the 11-15 age group. This is a time of physical sexual maturity ( puberty) which causes the most emotional distress in individuals due to physical body changes or lack of changes & evaluating whats causing the distress is an identity issue related to physical changes or the desire not to have any changes at all, or a conflict with sexual preference or conflict with societal expectations of genderbor all of them or even others.
given that this phase in physical development is pernanent physical changes of secondary sex/ gender characteristics, as well as over all physical growth phases, the ability to change genders & live as the opposite gender for an extended lenght of time as the 3-5 year old age group has , for intergration as the opposite gender , indentity wise & emotional wise with peer groups , parents, is far less time.
Puberty blockers could only provide an additional 6 months to 1 year of decision making before physical developmental processes has to be allowed to take place for normal physical development. that is application of hrt, or removal of blockers and allow a natal gender puberty. again its on an individual basis determination.
surgical interventions that are irreversable, that would be the most cautious route because even the 11-15 age group by legal age of majority , 18, there is a spread of 3-7 years of opposite gender living for which to make a decision that this is what the individual wants. The most assured case for irreversable surgery is the 7 years of life experience as the opposite gender. The least assured is the 3 years life experince as thenoppisite gender.