Given what is currently happening with that particular franchise, one that had been widely advertised with theatrically released and online trailer, do we really think Marvel will be touching upon that in the near future? Eventually, yes, I want to see a proper NM as much as any fan, but right now they are going to put spotlight on the X-Men realistically.
I would prefer an X-Men closer to college age or running that gamut myself, 18-24ish.
Casting and having the charters in a more high school teen age has advantages to that I'm not blind to, one of them, again, being filling the slot if you will, now empty due to the loss of Spidey (As of right now...).
Also, even if that were not the case and they went with a slightly older cast I still wouldn't expect New Mutants any time soon. I wouldn't think that would be on the table until at least a second or third X-Men proper film had been done and released. That way the team could be contrasted against the younger mutants.
I don't think the X-Men are going to be either already established or mature adult types, or that there's going to be a slew of mutant films right away. The former begs a lot of questions it is just easier to side step with a cast that's mostly under 25 and the latter is too pie in the sky at this early in the goings in my estimation. I could turn out wrong but I don't seriously consider the idea of non-Logan or Deadpool spinoff Mutant films happening until a second or third X-Men film is out there.
So I just don't see New Mutants proper in the MCU happening anytime soonish to be their go to teen series.