Don't really know much about losing it slowly but god damn do i know about losing it abruptly. At the age of 20, i'm coming out of my second ACL surgery in roughly 1,5 years(not from basketball,playing soccer in europe). Not sure if y'all or more interested in more of a basketball-related input but in my view it is like living with a life-lasting disease. I feel kind of bad comparing a physical injury to a disease but that's the best i can do.

You feel your body breaking down, and you find it nearly impossible to improve in anything revolving around explosiveness and pace. In the best possible scenario, you may manage to make your body somehow execute drills and movements at an approximate level of what you could before. This is one of the hardest thing for aspiring athletes to accept, giving up on progressive thinking and development(''Athleticism-wise''). It breaks a lot of athletes down, because if you don't find a way around it or some other skill to replace it with, you tend to get stuck in the past constantly trying to chase a version of you that you will never see again. It's really a very interesting process to witness and/or be a part of, mostly because players relying on a style of play with a lot of athleticism often explode during their teens. In an age where you might develop physically faster and more than others, you are able to dominate in most aspects of the game. Early on in these guys' careers it becomes a part of their identity and rarely do they give even the slightest thought into the fact that this aspect of their game isn't going to last forever. Newsflash, it won't...

Now, this is only in the aspect of athleticism. Great athletes have the mental strength and motivation to find ways around it, and know how to breakdown and rebuild their style of play to still be able to compete at a high level. MJ and Vince Carter are some of the examples in basketball, and you could mention Totti and Ryan Giggs in the world of soccer when talking about players that had to acclimatize to a body slowly losing its athleticism. So, for professional, successful athletes I imagine it being more of mental challenge, which really will breakdown some athletes(Shawn Kemp comes to mind, T-mac and Amar'e also but they were mostly injury-related), who will feel totally lost when not being able to rely on their athleticism, whilst others will find a way to age like a fine wine.