Who is making these sorts of specific predictions and why is that necessary? Has every great player ever been highly rated as a teenager?
https://www.fourfourtwo.com/gallery/ranked-50-best-teenagers-world-football
You need only look at something like this to realise it's all a crapshoot...
While there's a lot of baloney in that list by whoever came up with it - the top 10 were and are, with only 1 exception (Douglas Luiz), all excellent players and remain near enough the best players in their age group. In fact if you focus on the top 10, those who the compiler of the list would have most likely considered to be dead-certs, then there's a 90% hit-rate - suggesting it's not actually that much of "a crapshoot".
At 21 nobody really knew that guys like Modric, Xavi, Scholes, Iniesta, Pirlo, Kante, or Kroos would become the players they did. When each was 21, there were tons of CMs of similar age that were rated just as highly or higher.
Who are some of these CMs who were rated higher at the time?
The majority of those you mentioned were definitely already on the trajectory to being top-tier midfielders prior to 21.
Kroos was already a first-team starter at Bayern at 20.
Xavi became a first choice for Barca under van Gaal in a title winning season at 19.
Iniesta became a first choice at Barca at 20.
Pirlo made his Seria A debut at 16. There were a few years where he didn't make that big an impact but then he did enough at 21 to convince AC Milan to sign him for a sizeable fee (with him joining a month after his 22nd birthday).
Scholes - debatable - he broke through at Utd from 20-22.
Modric was more unusual in not getting a move to Spurs until 23, but his later rise can partly be explained by starting his career in Croatia, as opposed to a higher profile league. But he was still a starter for Croatia at 21, so wasn't a complete unknown. It did also take him until he was about 24 to start hugely impressing.
Kante did basically come from nowhere, not even playing in the French top flight until 23. But he's more a rare exception, than typical.
I also reckon with wider networks of scouting and increasing availability of data on lower leagues, rare rises like Kante will also become less common.
