I note that one of the two teams you were referring to above was Barcelona. While I agree they probably wouldn't spend £70m to buy Vuskovic, they have repeatedly shown that if he were already at Barca, they would almost certainly have played him.
I suppose it helps when you have a tried-and-tested pathway for young talent to break into the first team. Those of us of a certain vintage remember the likes of Xavi, Iniesta, Valdes, Puyol, Pique, Busquets and the Little Magician himself graduating into the senior side.
In the current setup, Pau Cubarsi, a central defender, made his debut at 16, established himself at 17 and now, at 19, already has two league titles and starts for Spain in a back four.
At left-back they have Balde, who established himself as a teenager. In midfield there's Pedri, who was a regular starter at 17, with Gavi not far behind. And, of course, there's the 18-year-old Boy Wonder on the right wing who was voted Player of the year in Spain, last season.
Barcelona's philosophy isn't perfect, but when they believe a young player is good enough, age and lack of experience rarely seems to be a barrier.