IF we didn’t impose gender before the age of consent, there’d be no reason for anyone to switch one before such an age.
OTOH, any logic that can justify the imposition can justify the change.
Puberty blockers are not, generally, irreversible (or even permanent-but-reversible, simply stopping taking them usually results in puberty), a fact widely recognized and documented in coverage of their use in delaying precocious puberty, long before ideological activists started attacking them based on a mix of anecdote and fiction because they were also used in treatment of trans kids.
The reality is that there are no long term studies on puberty blockers since in most used cases they are used as an intervention until the underlying medical condition can be resolved including in precocious puberty.
The fact that individuals can be on them for years or even a decade without any significant long term or permanent side effects also isn’t supported by evidence.
A successful HRT becomes much harder after puberty. On the other hand, performing HRT on children is plain unethical, for all the reasons discussed in the rest of this thread. (And btw something the trans community is advocating against - because forced sex reassignment had been standard practice for intersex children for decades. The trans community is demanding to end this practice.)
So to solve this, the idea was to artificially postpone puberty until the person is old enough that they can make a mature, informed decision about the subject.
Of course that logic has its own weak points - e.g. that puberty itself may play a major role in the person finding their identity. So postponing it until the person has settled into an identity would be sort of like trying to outrun your shadow. I'm not sure what the status of science in that regard is though.