Yeah, but it started in opera. I mean, it’s what coloratura soprano is, basically.
And, BTW, I did not mean to imply that the Vienna Boys Choir is opera. But I’ve heard them do solos, and I think they could do some adult opera parts, as long as they aren’t expected to sing above a full orchestra.
There are definitely opera parts written to be children, even if they were to be performed by adults.
Well, no, it didn’t start in opera. It predates opera by hundreds, if not thousands of years. It’s used in all sorts of styles of music, across the globe. If using musical devices that are also used in opera is what makes something “operatic” then we may as well end the discussion, since at that point more or less everything is operatic.
Castrati did sing female roles, but by the late 17th century, they were actually singing male roles. From wiki:
So because of a modern paucity of castrated males who can also sing opera, you now get performances of, say, Handel’s Giulio Cesare with a female playing Giulio.