Plenty of CB didn't develop that much physically the way Stones did, and while adding strength is probable, it is unlikely he will gain height and athletic ability.
There is also the point that
@Andresito made, development isn't linear for everyone. You can point to players who played their best at late 20's (very few are at their best at early 30's, it is usually end of their prime) but there is also early blommers. A guy like De Ligt looked even better than Cubarsi at 18, and at 26 there is 2 big clubs that were glad to find a decent sale for him.
So, it isn't a sure thing that he will be the best CB or even top 5 when he peaks. There might be players who are far worse now that might end up better, or not.