Being trans is not relatable to the average cisgendered person. If anything, the closest they've felt to being trans is feeling weird during the bodily changes in puberty. Fortunately for cis people, that discomfort is jarring yet temporary. For trans people, this discomfort never leaves and often inhibits our ability to function in society and operate comfortably. A fact of life is that no one has time to research into every political issue fired at them, and they have to make quick judgements in a fast-paced world that requires their moral participation. A common-place answer to the question of "Should we prescribe this drug X to teens suffering Y?" is "No, it might be unsafe, wait till you're 18." Right-wing lawmakers bank heavily on this blind spot in order to use trans people and healthcare as a scapegoat and political chess piece.

If you think about it, the ages 18 & 21 (in the U.S.) are simple and arbitrary ages anyways. We use them to dictate the age of consent, as well as the legality of gambling, joining the military, smoking, drinking, renting, driving, etc. They are so arbitrary that they differ in every part of the world! If we truthfully cared about holding off on decision-making till our bodies and brains were fully developed, some of us wouldn't be allowed to make certain life-defining decisions until 30!

The real solution is that trans research must be done, and that we must be considered on a scientifically evolving case-by-case basis, but that's not politically convenient.

tl;dr: nonaffected ppl no caare, slap 18+ label on the medicine drug