The Main team are supposed to be the Teachers of the Students, why would they be kids too?
Because that's exactly what they were in the first 18 years or so of the X-Men's publication history.
You don't have to get rid of the School, you can explain it with one line "I remember being a mutant as a child and how hard it was, I'm going to start a school to help them adjust to their powers/life". Done.
If you eliminate the correlation between puberty and the X-Gene as you suggested, then you would change the mythos on a fundamental level. Why would a school for mutant children exist when the X-Gene is not exclusive to children?
Plus the school hasn't ever been 100% open to only kids, wayward adult mutants have been invited to stay as well, Wolverine.
And at this point, they were all still considered students as there were no younger members for them to teach.
There's no rule that says they have to have been superhero's since they were 15, thus making them 10 year veterans at the age of 25. You can make them young adults that are just kind of drifting through life, struggling with hiding the fact that they have powers.
This is frankly impossible. A character like Scott Summers could not hide the fact that he has mutant powers for 10 years, not when you consider the fact that he barely has control over his Optic blast as an experienced adult. In fact, his powers being impossible to hide is the reason why Charles discovered him in the first place.
How does Warren hide his wings from society for 10 years when Charles isn't around to give him a corset? Does the tractor incident with Piotr not happen when he's a teenager or does his origin be moved up to when he's much older? And if so, Illyana/Magik has to also be aged up 10 years. And nobody has discovered Hank McCoy's ape-like proportions by now? All of these characters are discovered by Charles at a young age when they reach the point where they can no longer hide in society. So now we're suppose to believe that's what they've been doing for 10 years
The only way that someone like Kurt, Hank or Warren could from society is to go underground aka become Morlocks. Scott would have to stay in a reinforced Vibranium room for 10 years
Xavier used Cerebro to hide mutants in the Comics, easy fix for why they aren't known in the MCU.
Cerebro in the comics was created for the exact opposite purpose. To find mutants, not hide them. And when has Cerebro ever been used to hide dozens of mutants in the comics? And if these characters are NOT heroes at this point and are just sort of "drifting through life" not knowing their purpose, why would Charles be hiding them when he doesn't even know them? And if he's been hiding these people for 10 years since their powers first manifested, then this raises the question of why he didn't recruit them much earlier? Why didn't he take these people under his wing when they were young and needed his guidance more than ever? He would have seen their struggle and growing pains going through life trying to hide their mutation for 10+ years.
As for why they didn't show up when tragedies were happening in the MCU, think of it like the Army, there are plenty of people who have the potential to be excellent soldiers who just don't join the Army or join alittle later because they find a cause worth fighting for (alot of people joined after 9-11 who had no intention of joining beforehand). Xavier creates the X-Men as a team to combat rouge mutants, and as of now in the MCU, there is no mutants, thus they start at whatever age they are when Mutants begin to pop up and cause problems. It all starts when Xavier finds them, there is no rule that says he has to find them when their powers first manifest and the school really starts after the main team is formed, as they are the teachers. For me, I'd start the original team out as young adults, they start the school and then the teens from the school move up to eventually become part of the team. Starting in the sequel you can start mixing in the teen mutants, that way the whole cast isn't filled with "teenagers".
So in essence, instead of the X-Men being formed as a teenage group when their powers first manifest, they are now recruited by Charles when they are 25-29 years old, 10-12 years after they've been living with powers. This change is ill-suited for the X-Men. It's trying to avoid the seasoned X-Men in the first movie problem while at the same time time trying to evade the proper steps necessary to get to that point. It's like trying to make Spider-Man 15 when he was bitten by a spider, living with his powers for 10 years only to have Uncle Ben killed when he's 27 and THEN, he becomes a hero -- at 27 years old.
For the record, I'd make them young adults too as even in the original run, they graduated High-school fairly early on. However, I'd make them be YA on the
edge of adulthood. People who are fresh out of High-school and like all of us at that stage in life, are still finding themselves. Trying to figure out "Who am I?" . Each of these kids (17-23) come to that conclusion in a different way. Each of them have a flaw/insecurity that they have to overcome. That makes for a very compelling arc.
All these "problems" you mention can be easily worked around, most with a single line of dialogue. And you can always use flashbacks to explore their struggles as teenagers. The bigger issue is how you explain Magneto being a Holocaust survivor in 2020 or Wolverine being in World War II. But that's a simple fix too, they age slower or not at all due to their mutation.
But the explanation you provided doesn't make much sense. Flashbacks do not solve the problem of an inherently flawed concept that is born from a desire to get away what the movie is suppose to be. I'd rather go on that journey with these characters and see how a young person deals with being a mutant and having growing pains while at the same time dealing with the grim fact that humanity hates them simply tor being.
Wolverine needs no explanation. He is extremely long-lived courtesy of his healing factor. That's been the canon explanation for decades.
Magneto needs to be updated
It's all very easy to explain away and doesn't cut Marvel off from any actor over 25, when all the best actors are 25 or older. And they have way more than enough time to develop their characters, regardless of age, that's all due to how much focus they can give each member during the run time of the movies. Age doesn't limit character development and they would all still be in their 20's besides Xavier, Wolverine and Magneto, who should never be teenagers outside of flashbacks anyway.
You cast actors that fit the character, not vice-versa. If a 19 year old Scott Summers calls for early-mid 20s, you don't make Scott 29 so actors in that age-range can play him.
There are dozens of accomplished young actors under the age of 27 that are taking the industry by storm. Of course you aren't going to find many 22 year olds on the same level as Meryl Streep but that's not what the role of an X-Man calls for anyway. Most actors in general aren't on her level.
Talent is not solely measured by age. Lucas Hedges is a better actor than Scott Eastwood. Tom Holland has more range than Joel Kinnaman. Hailee Steinfeld has more dynamic acting prowess than Jessica Alba and Lindsay Lohan combined etc.
Truthfully, you should be asking why Xavier would put teenagers in harms way in the first place. None of them should be under 18 while in the field.
And that's actually an interesting question to explore, in-movie. Especially in later movies when the X-Men ARE mature adults and start pushing back against Charles more. What right does Charles have to turn these kids into soldiers to fight for/further his "dream". What kind of moral and ethical questions does such a conodrun raise? How does that affect a young person's psyche? Etc. So many story avenues that they can take this franchise with a PROPERLY developed X-Men
