Can you give a justification for either of those things? Would you not let your older child be on your phone plan or drive your car under your insurance 20 years ago because both of these things weren't really trackable back then? If you would, what changed? I simply don't get what 'lesson' this is meant to impart on them.

First, I responded to examples of 23 and 29 year olds. Second, I pay for my adult children's phones and don't track their locations, but I would if they asked me. They have lost phones and mentioned that they should at least setup find-my-phone and give access to someone. I would let them track me, but I'm still using a flip phone.

Vehicle insurance and owner liability is a whole different topic. I prefer to have them hold title and therefore liability, even if I were to give them the money to purchase and insure.