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