Probably because it’s much harder for adults to change their identities.

They have lives, responsibilities and dependants. Unlike children they can’t change their identities on a whim without repercussions, or negative impacts on people they care about.

I imagine that many people would rather ignore their “true” identity, than risk losing a spouse they care about, or disrupting their children’s futures.

We do? I've seen more adult trans people come out in the past year than the entire decade of 2000-2010.

> If social stigma is no longer an issue,

Who said that? It's probably becoming less of an issue, but it's definitely not disappeared.

Sure, but are you implying the remaining stigma applies unequally and is dependant specifically on age? Is there any evidence of this?

It certainly wouldn’t surprise me if adults with decades of experiencing living with that stigma would choose differently from young people without those decades of experience.