Mate, apart from the Sanchez signing, most of our business has been exactly what you're describing:
Pogba - massively talented yet still fairly young when signed, we knew he still had to develop
Bailly - early 20s, massively inexperienced
Lindeloff - same as Bailly
Lukaku - 24 when signed (I think), still has learning to do
Fred - 25, still has learning to do
Dalot - 19, massively talented and inexperienced
Then there are the players we already have such as Martial, Rashford, Shaw, and Lingard who are in the same mould. This team has tonnes of learning to do. While that theory of yours sounds like a great idea, it's very evident that fans aren't quite willing to be patient for all of these players to click.
What we need is immediate improvement and results. If that means signing a number of 28 year olds who will lift the team, then so be it. We look to sign young talents every season or 2 anyway, and that never stops no matter how successful we are on the field.