Transitioning as a physical, medical procedure is only relevant immediately preceding or during puberty. Before that, sexual dimorphism is quite limited - kids assigned to either gender at birth tend to be quite similar to each other in most physical characteristics (except for the obvious), and if a child of that age were to "transition" it'd be limited to the realm of the social - things like clothes, names, pronouns, and haircuts, which could easily be reverted anyways. I am 100% in favor of allowing minors to undergo medical transition once they reach the relevant phase of their life cycle, and frankly it should be an obvious conclusion. You wouldn't keep someone from getting treatment for a broken leg just because they might "change their mind" about it later, would you? It's a serious medical ailment, not a flight of fancy. The consequences of waiting until after puberty has been allowed to run its course unabated, on the other hand, can be positively disastrous. By that time there will be many physical differences which have formed that need to be painfully undone, in ways which are significantly more expensive and may not give the same results as having taken action earlier, not to mention the lasting trauma of being forced to live in a body you hate. If you break your leg and you don't attend to it, it's only going to get worse. It could cause internal bleeding, or get infected, or shatter further without proper support, and it's only going to get harder and harder to address the longer you sit on it. The other responses here go into great detail about why the actual mechanics of the process are hardly the irreversible jump off a skyscraper many love to make it out as. Although this issue is most acute in the area of gender dysphoria, particularly in terms of frequency, legislative action, and severity of consequence, it is also a significant issue for those who deal with other mental issues, where physicians refuse to grant a diagnosis or proper treatment to those suffering from them who aren't adults.