I think you can ruin a rookie QB, but I don't agree that playing a rookie QB early ruins them at all. Hell, start that rookie Day 1. Not everybody has a Favre/Rodgers situation. Making a rookie learn the job early, like any job really, is not a death sentence. The over-achievers will prosper.
What I do think ruins them is the indecisiveness and paranoia from coaching staffs/owners/whatever that causes rookie QBs to get pulled two games after you start them. Who then start them again two games later, and then rinse, repeat, etc. That seems more reserved for project QBs taken too early (EJ, Kizer), and it just does not seem to ever work out.
Allen is raw, but is not a project QB. He's an A+ ideal mold of a prototypical 2018 QB, so you just can't pull him after starting him in fears of ruining his mental. You need to just keep running him out there at this point. That shouldn't ruin him if he's a true #1, but the horrible injuries that he may sustain behind this line definitely could.