Matuidi went for 7 million euros to PSG aged 24. He was the least flashy option out of all the french midfielders that the NT coach was considering for the 2012 euro which he then missed due to injury.
He improved steadily ever since at a higher rate than his time at saint etienne. Late bloomer. You don't need to be crap before that to be one.
Similarly, Valbuena impressed rather late, no one would have thought when he turned 25 that he'd be the best french NT player post 2006. People are actually not coming to terms with that to this day.
Not gonna discuss Zidane whose best years have been in his thirties.
The point is, some/many players improve significantly more well into their twenties while others stagnate.