Lol. This discussion.
Luffy is a childish protagonist, much like Gon or early Goku or Naruto (first part) who act like 12-14 year olds but have to deal with adult problems and hence is forced to make decisions adults would make whilst with child like behavior. That's it. Most protagonists aimed at the Shonen demographic does the same.
The difference between early Goku and Naruto to Luffy is the age. Luffy before time skip is 17 while Naruto was 12 and Goku started off at 11. Gon is 12-14 or something. This is the reason why people call Luffy more childish than most other protagonists.
Character wise -
Luffy stands out for how stupid he is, Goku for how naive/stupid he was/is, Gon for how innocent and naive he is.
Midoriya doesn't act like a child. He acts like a late teen and tries to make the most logical decision he possibly can in most scenarios. Tanjiro is the same as well.
Please tell me I don’t have to be concerned about act age rating
You don't. Its a pretty unconventional manga in WSJ with growing sales. It'll be fine just like CSM which keeps lingering in the lower half most of the time.
-The groupal cover for me gives another message that they are done with Black Clover
Shhh, making logical observations about BC from magazine placements can lead others to think you just hate BC and are making illogical claims.