How much endurance do you have to lose when you age?

https://www.trailrunnermag.com/training/most-people-get-slower-with-age-but-is-that-inevitable/?_hsmi=255642873&_hsenc=p2ANqtz--6KtgOZltwQB7Vy4uoEVJ8a7CINBOAwYr6PF7POoACEVyrEiMucbwfyoiHaWXy6ImZhAUIBWqQ75NDWgyN8gCFopqQJw