Alien 3 was in 1992 and I don’t think Mulan did it out of a Crisis situation and more of a woman passing as a boy trope, which is older than TV or cinema.

Final Fantasy 9 did it, arguably.

The story of Mulan seems to take place during the Wei dynasty, and was probably written during it as well. So that would place it between the late 300’s and early 500’s CE.

I immediately thought of Samson from the old Testament. It’s estimated that story takes place around 1,000 BCE. Not sure if anyone knows of anything older.

It happened to a guy in And Justice For All in 1979.

the legend of Billie Jean was 85’