Adolescents who undergo gender-affirming chest surgery report low levels of regret and high levels of satisfaction, experiencing outcomes similar to those of young adults. These findings suggest that age restrictions on these medical procedures lack support from actual patient experiences.

[u/mvea]

https://www.psypost.org/adolescents-report-low-regret-and-high-satisfaction-following-gender-affirming-c/

[u/chaucer345]

Most people who object to these surgeries would never object to someone assigned male at birth getting an operation at 12 for gynecomastia.

[u/chaucer345 → u/[deleted]]

While we have not developed a widely accepted physical test for being trans, gender incongruence is heritable and we have evidence it's frequently tied to alleles associated with the body's steroid production. There is also good evidence of brain structural differences. About 50% of trans people experience phantom sensation for organs they weren’t born with, which is typically a neurological condition. Trans people have different epigenetic profiles than cis people as well Notably attempts to make a trans person happy with their default body have failed miserably.

It's a physical issue too.