Then why does the dwindling inner reserve seem to line up so neatly to completed objectives? E.g. you successfully emerge from the dark period, but are unwilling or unmotivated to grind through another. The end of a struggle is not usually conveniently timed to match your age.

One explanation is the need to complete the objective compensating for age.

I agree, although I don't know that it's specifically a chronological function. I'm starting to think of aging as just an accumulation of abuse and trauma. Maybe that's physiological - sun exposure, broken bones that didn't heal quite right, whatever. But it's also mental. The parent comment is a great example, but there are basically an unending number of them. Loss of family or friends. Financial struggles. Mental illness. It all just adds up.