Transgender - How soon is too soon for the transformation?

So… there is a child who attended elementary school with my son. Somewhere along the way the parents choose to home school for awhile. Now, the child is a freshman at my son’s high school and is currently undergoing procedures to change her into a boy (and no, I don’t have a problem with that).

Having known a couple of transgender adults along the way who made the transformation from male to female, I totally get the whole difference between gender identity and sexual preference and understand that there truly are people who have a gender identity that does not match their physiology.

That being said, how soon is too soon to make such an irreversible change. On the one hand, if one is truly trapped in the wrong body why not make the change as early in life as possible. On the other hand, what early teen truly is comfortable with their sexuality and gender identity?