You are missing a key apostrophe there, "'", which grammatically means that ambition belongs to fulfillment.
The author is saying that as he ages, the fulfillment he once felt by being ambitious (e.g. writing books, starting a company, learning new things) goes away.
Does that make sense?
I think it's important to keep in mind that this isn't a "factual" piece as much as it is his reflection on aging. It might be true to him but not true to others.
Ambitious people set high goals for themselves. High goals usually take time and energy to achieve. When you're old, you are on short supply of both, and so a person ambitious in their youth can no longer afford to be as such at say 85. You've only got so much time left, it's not worth it to be a prospector.
The author likens this state of affairs to that of an aging athlete who can no longer play the sport they loved playing in their youth. For an ambitious person, settling for a slower pace can be a difficult transition.
Unlike the others, I took it to mean the loss, the gap in one’s life, after their ambitions are fulfilled. If one has led an ambitious, successful life, their early life is filled with problems and purpose. Later years have lost that purpose, even, and perhaps especially, if you’ve been successful.
> Anyone ambitious who lives to be old or even _old_ endures the inevitable loss of ambition’s fulfillment.
I needed the whole sentence to parse that phrase. Here is what I understood: Ambitious persons who live to be old will struggle with the realization that although they remain ambitious (a certain type of stretching into the future this characteristic of _youth_), they lose the ability (energy, focus, commitment) to fulfil those ambitions. Matching those two conditions found inside one's self as the reality of one's self is a terrible struggle and out of that follows an inevitable sense of loss.
