Adult onset mitochondrial disease can happen. Mito is a term for a wide range of diseases of varying severity, and as research discovers more, a mitochondrial element has been identified in existing illnesses, or is being explored eg with ME.
It’s a bastard disease because it can strike ‘any system any time’ as our consultant used to say. Many adults have mild mito or metabolic disease and never know it. Others develop symptoms in adulthood and die but that’s far rarer. Even if you know you have mitochondrial dysfunction you may be totally unable to predict how it will progress.
It can present as regressing/remitting like MS, but a person generally doesn’t regain all lost ground. You could possible describe someone as ‘getting better’ in those circumstances.