It's not that youth is a catalyst for success, it's that it allows you continuity when developing a squad. When you buy proven post prime players, you are normally paying them high salaries, big fees and you have a very small window of time to achieve things before they have to be replaced. This is fine under certain circumstances e.g. you are a complete team with one/two glaring weaknesses that need to be addressed.
When you buy younger players, they are typically paid less and there's a good chance they improve as players and have time to gel with the team's chemistry. If they don't work out, they can be sold. As bad a buy as Van de Beek has been, we should still be able to recoup half his value which can be reinvested back into the team. When Casemiro's contract finishes, we will likely get none of his €70mn transfer fee back which isn't a sustainable way to do transfers over time especially when our entire spine (De Gea, Varane, Casemiro, Bruno, Weghorst) will require replacing all at the same time.