Okay so not as a general rule but as a lot of people I've noticed...
It seems that transmen pass best as looking younger. Ive seen teen or young adult males pass as little boys or adolescents, Im assuming this must be from the size and rounded facial structure. Even older transmen seem to pass as younger than their age.
Whereas it seems the ladies pass when their older assuming they still have masculine traits., I've seen a lot of cis older woman have masculine traits. Something about the oncoming signs of age with wrinkles hides the facial structure, a lot of them tend to have short hair even, andsome of them have deeper voices. I'd say some of them if they weren't wearing feminine clothing I wouldve mistaken them for little old men.
As always, not everyone follows that general rule. Transwoman who have round faces or are naturally androdynous pass as their age, and most men on T catch up with muscle tone, voice, and body hair. I just thought it was a little funny thing to notice.
Probably because of the skull. The mature skull of someone born XY is typically
* larger than female
* longer than female
* has a larger and generally squarer forehead with more prominent brow ridges or visibility of edges at the temples
* has a wider and squarer jaw
Since most of us didn't have this type of development we likely look smaller and have more rounded skulls, even after T has done its work. As you say that lends itself, along with generally smaller body size to the assumption we look younger.
I've only just stopped being carded at mid 30s - I am particularly neotonous. Anything that reminds people of a child's face will do it - in my case it's because my eyes - for some reason look quite big in my face and my nose doesn't dominate my face. I've still got people commenting that I look ridiculously young. I'm not sure if facial hair with that would be a nightmarish combination, haha. In any case I typically fit better into a 20 something presentation anyway.
Oh, I don't know... I'm in my 40's, was born with a large head... I have no issues passing at this point. I've also seen gals with huge heads and guys with tiny ones... I think it depends on other factors too. Brow ridges, large shoulders, unibrows, hair in bad spots... it all adds up.
i wouldn't mind looking a little younger than i am, key words "a little", as i get older. though considering people say that about me already as a "girl", i consider it a bad sign for just how much younger i'll look as a guy... there's a limit, you know. on one of the occasions where i passed, the person thought i was my ex's son. it was weird, because i'm actually a little older than him. sure, in the future i wouldn't mind people doing a double take and going "wow you look great for 60!" but until then, i dread being called "cute" or mistaken for the child of whoever i'm out with. it's pretty degrading.
I've got a cis friend who does look 16-17 but is 31. He's never said people treat him like a child as far as I know.
I've had to show ID when I buy tobacco or alcohol. That is nice when I'm almost 40. It's hillarious to watch their face when they see my age on the ID.