>>10765321This was their kid/preteen look(the title of that issue even calls them "Preteenage Mutant Ninja Turtles)
Also while it's dubious canon, the turtles got the idea for their masks from their universe's version of Robin. I say dubious canon because it's not a guest era issue but a guest starring issue they had that was sanctioned by Mirage and uses Mirage continuity.