I think 25, 26ish is when a player is no longer 'forgiven' for his flaws, but before then, things are generally expected to get better. I'm not saying players morph from one form to another, but that they can work on their weaknesses and carve out a style and niche for themselves and polish off certain aspects of their game. Do you remember the pre-season Rooney went away and worked on his heading? He came back one of the best headers of the ball in the whole league despite ending the previous season as nothing particularly special with the ball needing to be headed. Even if he's an outlier, there are aspects that you would surely hope to see honed if a player works hard enough on his game. Wan Bissaka, with intensive training on his passing, one-touch and crossing should see improvement - you drill enough and a breakthrough of sorts should materialise. I don't think players are done and ready for the scrap heap if they're not showing much at 22 or so, personally.Yeah but we signed him at 22, on a first-team transfer fee and wages. There's not that much room to add that much more in terms of technique to a player at that point, most of the improvements are about consistency and awareness. A player might add specific skills that they didn't use before such as backspin long passes, more weak-foot play and the likes, but they don't massively change how they perform some of the basics.
Think even about like a Patrice Evra who was signed at age 25. He didn't have any massive improvement in technique over the following 5 years, he had a good enough base level and then massively improved his consistency and awareness, while some of his limitations (one-footedness) remained pretty much his entire career.
I honestly think that in some clubs and countries AWB wouldn't even be in an academy team in his late teens with how unnatural he seems with the ball.
Having said that, Wan Bissaka isn't 22 anymore and there may well be not much he can do to develop from here on in, but I'd like to see what becomes of him under a strict coach over the remainder of the season - if he's exactly where he is now as a player, I'll be surprised.