In soccer U23 is still considered fairly young. Hence in internationals they have U17, U20, U23 and open senior comps.
This is why Roos has such a good point IMO.
I myself was a very late developer and whilst I played some good jnr footy as a young fella by the time I was 15 or 16 I was still a pretty much a boy playing against men in many cases.
At about the age of 16 I finally started growing but I didn't start playing really good football again until I was about 19.
The skills were there still but I was just pushed off the ball too easily and didn't make an impact again until I was more physically mature.
The same could be said for many kids aged 17 or 18.
Some have an unfair physical advantage as juniors but are found out at senior level when they are playing against men as strong or stronger than them.
Alternatively there are some very good footballers out there who get bypassed because they haven't physically matured yet.
I honestly think you would see a fair difference in where many players get drafted at 17/18 compared to their order at 19/20.
When you are talking about a 10 yr career it can't hurt to wait one extra to see who's the best when it's on a more even playing field.
Roos is spot on with regards to this IMO.