But that doesn't mean it should be, does it?

An inconsistency in perspective only means that both perspectives cannot be justified without additional refinement. One could be just plain wrong; or the other could be; or both could be; or both could be right because there is a key discriminating factor that has not been appreciated. But certainly both cannot be right for the reasons offered.

As a practical matter, if someone in their 60s adopts the dress, behavior, and attitudes common to someone in their 20s, there isn't that much resistance to accepting them as "one of the crowd". I've seen it happen multiple times, when the older person really goes all-out to "be young" to the extent possible.

For medical issues, they still need to fill out their age, and yeah, we don't have separate categories for "chronological age" and "age expression", but that's mostly because it isn't actually a major source of discomfiture, isn't it?

The parallel would be that a trans woman could call herself a woman but not female. A trans man would not be male. Which is pretty much how it goes already.

So I don't think your example shows what you think it does. There isn't that much stigma against transage identity; it's a bit weird but only a bit to be "young at heart".

The stigma against transracial identity--despite race being a silly largely made-up category--is pretty strong, but why should we infer anything from that? If the identification is deep and abiding and important to the person, and they take steps to "be" that race (hair styling, maybe changing skin tone, etc.), why exactly is it critically important that we shame and mock them, and police this categorization?