The meaning of US old-growth forests is slightly different from what one would first think. The US has about 820 million acres of forests. Out of that about about 144 million acres are under the management of the US Forest Service. Out of that, 25 million are deemed old-growth, and fall under this proposal [1].
[1] https://www.fs.usda.gov/about-agency/newsroom/releases/usda-...