It's probably not fixable if it's really that. Even if mtDNA is fine the mitochondria have their own mechanism for senescence based on other damage, just like cells.
It does not even have to be caused by radicals.
And would there be a way you think to test if that's really the case?
I have talked with some people who had like severe reactions to the drug who managed to get some kind of mitochondria testing and it would say stuff like "30% of the active sites are blocked on the mitochondria"
