When to decide on cosmetic surgery that results in sterilization?

Unless one is really really distressed and mental treatment has been determined hopeless, probably never.

Otherwise the question is simply when to choose cosmetic surgery.

Disorder is defined as that which inhibits or interferes with any normal function or established system.

Ours is mating. Pairing and joining to reproduce. Reproduction is a continuum where living cells from biological heterosexuals combine creating a new individual sharing the genetic history of both. No other gender relationship can achieve this fundamentally unique process necessary for life.

Look at the history. It was bullying not science tha persuaded the APA to remove homosexuality from the DSM.