I don't necessarily disagree with you there, but I also think being on a lease is one of those first 'adult' things that they should be exposed to. After all, most people do get an apartment sometimes.
I have seen way too many instances where young adults have no idea how a contract or lease works and they make big mis-steps. Better to have the first lease with mom or dad than solo. Just my view.
But the other side of it is that the kid is now an adult and as a landlord, I would feel the need to address it at some point. Since the OPS tenant is a month-to-month, I don't think two or even three months is an unreasonable time frame.