It's not the age it's the experience, young players are inconsistent, this is just a fact and always will be, the frontline you have there would struggle to get top 4, they'd be of no threat to City and Liverpool.
Look at the top sides in Europe and show me a single one feilding a bunch of youngsters as their frontline, there aren't any as you need a blend, you need a couple of proven, experienced, in their prime forwards to help the younger one out, to let them develop alongside forwards that are ready to carry the load.
You mentioned de Ligt, I'd say he is an exception, Captainining his club at 19, that's rare, but it's also a different position, scoring goals week in and week out at the top level is a different task. On Mkhitaryan and Lukaku, one lacks the mentality and the other was never good enough, the fact we bought them isn't an argument against quality experience, it's just another check in the column of why we need a DOF, so the right experienced players are bought, not ones like Lukaku and Sanchez.
Man you hate Neves almost as much as you like Milenkovic.
I wouldn't chance it on either of those over him personally, but he's not a holding midfielder right now, he plays in a double pivot with a back 3 behind him, if he played here it would be more as a #8 with someone else holding IMO. The thing is I think we do see a lot of posters being against signing quality because it might get in the way of a 17 year old who hasn't played a second of senior football, we are already seeing it with people not wanting to buy a holding midfielder because of Garner, it's part of the Academy obssession that the class of '92 bred into people, but people forget to look at the quality we had for them to lean on when they came in, the club is not at the point right now where we can accomodate a bunch of kids that just don't have the experience or nous to take us where we need to go.
Jovic has had one good season, he looks good from the games I've seen however he does come with one of the same issues as Rashford in that he needs a system that doesn't require him to hold the ball up, as that is a weakness in his game, he can run with it but the link play isn't great from what I've seen.