Making the 10,000+ year old have the maturity and personality of a middle school kid going through puberty wrecks the story. It makes it cliche, boring and cookie cutter. It ruins the well set up overpower character idea. He can't get what he wants through power, IE a normal and carefree life.
If the author wants to go with having the awkward kid, but tens of thousands of years to gain his nigh unmatched power that is fine, just give a reason for his personality reset. Luckily there is a perfect one that reinforces the actually given story. The Most powerful demonlord wants a normal life.
If the Main character intentionally reset his maturity and potentially some memories to that of a middle schooler just starting puberty that would work as an explanation. That is just the point of "normal" the demon lord wanted to start their new life from. It reinforces the "wanting to experience a new normal life" stated goal of the story and characters.
Maybe it is a dumb reason but it would make suspension of disbelief for the MC's maturity easier by giving a somewhat logical reason why he acts the way he does.
It sucks cause the rest of the set up is a solid enough twist. If you want an extremely OP MC give a goal or goals that can be simply achieved by their OP abilities. In this case, the Demonlord that was born to be strongest wants to have a life not built around being the strongest villain. That can't be directly or simply achieved by power.
That Time I got Reincarnated as a Slime is one of the best examples of this idea executed well. The town and it's people can be hurt, Rimuru can barely be defeated in combat.
The Misfit of the Demon King Academy 1st season did it as well. Manga and originally story less so. Anos Voldigoad carries the story alongside the 2 sisters who near instantly(first episode and 4th episode) acknowledge his extreme power. Anos wants a world of peace and has near omnipotent powers but the story isn't about his power, just how he shapes the world around his will with comrades/love interests/bro-mance.
Eminence in the Shadow does the OP MC by having the main character be the embodiment of players in a table RPG game ruining the Dungeonmaster's story. Shadow comedicly and constantly ignores the story and NPCs of the DM. The twist is there is a serious story being told by all the other characters. Shadow is just Haruhi Suzumiya and effectly in a completely seperate reality from everyone else.
/rant