A need for laws to prescribe what constitutes transgender

I am not an expert on matters related to transgender. As a classical liberal here is my statement on the subject.

I am afraid transgender is an idea whose time has come. Transgender will most likely not fade away or become a footnote in history anytime soon. If history is any guide, we have tried to ignore homosexuality for a long time. We may need to start to deal with transgender in Western countries.

Perhaps, some Western countries have already passed common sense laws to define transgender. I am personally not aware of any such legislation. However, such laws could serve as a prototype or model.

Not to pass any transgender laws e.g. to avoid giving transgender any legitimacy or to hope transgender is only transient is probably not an option. It will probably only make things worse or awkward! 

Has an individual legitimately transgendered when this individual declares to be transgender or are certain requirements beyond mere declaration to be met? What are these requirements?

At what age can a human being choose to transgender? At any age? What about parental consent? 

What is transgender treatment (also euphemistically called care) and who pays for it?