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 .
But thats a myth that you dont know what they can do until they play, we dont know because thats where we get to see them. If we could see them in training every day we would have a much better idea of if a young player was ready and I'm sure there are plenty of the u23 squad we could look at and say that player isnt good enough/ready to play for the first team without them getting first team minutes. There is a massive clamour for Lookman to play, if we saw him train and not look ready there might be more of a feeling that he isnt good enough yet. (theoretical if that is the reason why he isnt playing of course..) And that is before you look at what they are doing in u23 games themselves.
There is an element of you dont know for sure and people might surprise you when they get picked, as I'm sure Davies surprised Koeman when he got his chance last year. But you are implying the only way of knowing if a young player is ready is by playing them, which is wrong.
Also how many players play in the premier league 16/17? It is very rare that players get played at that age because they need to physically able to cope as much as anything. Plus the point about Davies as I said it is easier for managers to use young players in midfield or attack than it is at centre half as shown by Davies coming off the bench a fair bit before getting the regular spot he did last season. At centre half you dont get those opportunities to get some minutes, impress and eventually get a chance to start some games.
And the point about plenty of 16-17 in the lower leagues, probably easier to loan him out and get some league experience rather than just throw him into the premier league in a tough position. You see people like Galloway who is of a decent level gets loaned out to the championship rather than stick him and see how he does here.