At what point should a person be responsible for their own life? I wasn't taught a lot of things; I learned them with some basic googling. You can figure out how a credit card works with a few minutes of critical thinking followed by "I should look this up". Something I would expect a 30-40yo to be able to do, no?
It would be surprising to hear that someone obtained such a high line of credit that it took 20 years to pay off, while not understanding how credit works, but I guess that is possible