It's a strange argument considering that few of the current midfielders are young.
Pogba, Fred, matic, mata, lingard are all experienced. Pereira is 23 and Scott plays quite mature and doesn't have any problems with consistency/pressure.
It's a problem of quality not age/experience. 5 out of those 7 are squad players at best and that's already a very generous assessment. These are also the players who get most of the criticism.
There are a couple of players in other positions who are young/inexperienced and shouldn't have to shoulder too much pressure, but they are not the problem. James doesn't seem to struggle with it and the others are not really regular starter. One also has to accept it would be a success if 1-3 of the youngsters (tfm, tuanzebe, Williams, gomes, garner, Chong, greenwood) end up having a decent+ career for united. On average the majority of them won't make the cut simply because they are not good enough regardless of pressure.
United need to fix a huge hole in a midfield, that lacks the quality to transition the ball forward or to create anything while being not particularly good at pressing/defending.
It doesn't matter if you buy a 19 year old or 30 year old player as long as he is good enough.
Rakitic would add something and makes some sense, but I have the strong feeling that he gets overrated quite a bit as well.