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I believe the 100km Karman line is what is used for treaties and such. Which ya is kind of nebulous since you can find sources that'll say the Karman line is at 80km.
In any case, airspace is often a de-facto area of international law. China can say "you're not allowed to do that!" All they want. But the US did it. >

Airspace is not international law. A state has complete and exclusive sovereignty over the airspace above its territory. It’s why the balloon was shot down over the coast in American airspace. 
A quick google search can confirm this. >

Yea, isn't your airspace as high as you can enforce? Or something like that >

There's no firm upper boundary so far, except insofar as objects in orbit are not considered to be within any nation's airspace. >

So roughly the Karman line, which is itself not precisely defined.