We don't as fans know or find out because we don't see them for hours every day on the training field. I've no idea if he is good enough or not, but there will be plenty of people watching him and the others every day who see what he is good at, what he isn't, what areas he can develop and also what his attitude is like, what personality is and how someone him or any other youngster might cope physically and mentally with playing top level football at a young age.
There is a huge amount to consider about when a player is ready to play top level football. And that's before the fact that centre half outside of keeper is probably the hardest position for a young player to come in because you can't bring them off the bench to ease them in, and the mistakes that get made costs games so it's harsh for them to come into and it's hard for managers when jobs are on the line to do it.
Stones was an anomaly to be as good as he was at 19, despite how people's views seemed to drop on him later in his time with us he was and still is a world class talent in that position. It's easy for us as fans to just say play a young player because if they play badly, affects them mentally, loses us games, hinders their chance of a successful career because they aren't ready there is nothing on the line for us making that call but there is for the manager, player and staff but they have the best view because of working with them over a long period of time.
Typical of your posts you are stating the obvious ,sorry ,not so obvious is the fact that lots of players start their first team careers at 16/17 ,just lower down in the leagues .Tom Davies is barely older than him but that is fine -If he is good enough he is old enough -the only way to find out is to play him .The comment about the training ground giving the management all they need to know is wrong ,no one can be sure until they have to do it on the pitch .