The point is about playing at professional level and in particular a higher level than reserve leagues, it's not rare to see 18/19 years old players around Europe and England play on a weekly basis in first and second divisions of their countries, I would say that it's fairly normal. And players that are at big clubs and aren't playing by that time often leave their clubs for playing time permanently or on loan, in the case of Mejbri it's a good thing that we extend him soon.Of course. Thats just basic probability though.
The chance of an 18 year old being ready for full time top flight football is relatively small, then even smaller for a central midfielder, then even smaller again that this player is already at a big club and they have space to play him.
So yes, the majority of top players don’t come from top teams, but that’s not solely down to playing time at 18.