"At the time"
It isnt the time its 2019 and its 60 something years after the invention of the x-men with 60 something years of history to draw upon. If its something the MCU does well its draw upon the entire history of characters.
"All I said was that the original X-men were all in their teens. There were no older X-men until later." Apart from Xavier and magneto and mastersmind (yes technically not xmen).
By brand new xmen I mean brand new not new versions of existing ones.
"Not really that drastic. At that time the X-Men were created mutation was explained as a relatively new thing...so most mutants were in their teens or twenties. Yes, there were a handful of older mutants like Xavier and Magneto...but the thinking at the time was that it was really kickstarted at the beginning of the Nuclear Age. Mutants existing for thousands of years was an idea that wasn't thought of until much later."
Everything you listed there already exists its not the first time round this time. This isnt at the time this is now.
The MCU does not skimp on explaining brand new charters in their universe they usually give them at least 1 movie to their own and unless their xmen are created like ultimate xmen there is no backstory to any xmen if you go with the snap route.
Even if you went the snap route and you followed your route by going "at that time mentallity" you would just retroactively add history in which you cant because mutants were created at the snap were they not? Which in turn would make xmen much later.
You literally cannot have any history of mutants prior to snap if you go this route which is silly.