It really seems that, to a extent, it comes down to the user. More specifically, it comes down to what the user believes.
As we all know, Persona's and the World are both products of cognition. As such, it stands to reason that one's usage of a Persona is, for the most part, limited only by one's cognition.
Ergo, if, say, the Persona 4 cast does not believe that they can use their Persona's in the Real world, then they will not be able to use them in that setting. Inversely, the Persona 3 cast should, for the most part, see no problem's with them being able to use Persona's in the real world, Ergo, they likely would be able to.
The matter of a Persona User being allowed to use or not use their Persona in the real world all comes down to cognition- Much like other concepts we've seen, wherein enough people may believe one thing is real, ergo, it becomes real, such as The Rumors in Persona 2 (Surprised I'd ever find a way to reference the game that I'd actually understand), or in a more recent case where the inverse idea is applied, when Yaldabaoth uses the masses, twisting their cognition to erase the PT's- When enough people believe something isn't real or can't occur, it stands to reason that following this logic, that thing would cease to be or cease to possibly occur.
Hence, if The Investigation Team believe that outside The Shadow World in the TV, they can't use their Persona's, of course they'll be limited to just a group of teenagers. The same would apply to Joker- It would all revolve around his mentality. And while he is arguably on the higher end of understanding in the group when it comes to cognition, not to offend him or other's who understand cognition in the game well such as Futaba, but, it seems that it never crosses the mind's of any of the Phantom Thieves that they could use their Persona's in the real world.
Of course, this could be somewhat or even massively incorrect/wrong/misguided, etc. etc., but that is what the Series and the breadcrumbs it leaves has guided me to seeing- That Persona's and The World are made of Cognition, and that through said cognition is what determines if one can or cannot use their power- If they believe they can, they can. If they believe they cannot, they cannot.
Regardless though, I would like to remind people- As there are certainly going to be a few of those who will try to use this as a means to have Persona User's be relegated to a bunch of powerless teens- That for the means of the VS Battles on this site, it is assumed that said character's are in a state mentally and physically where they can use their abilities, anyways. So regardless of if Ren is only effective in The Metaverse, he will still be using those power's in Vs Debates, as there would be little to no other reason to have him in Vs Debates.