Basically younger players will train a lot better and can last for a few seasons, whereas older players will hardly train that well and will likely be quite poor within a couple of seasons.

As an example, I currently have one player still playing regularly from the second season when I bought him at 18yo in season 2. I am now in season 6 and that player is still around just by training and playing matches. Hell I even got him a sepcial ability (one on one goalscorer) without knowing about power training and he should hit 5* shortly.

Simply put, younger players can last longer if trained and play regularly, whereas older players are fine for the short term, but poor in the long term.