Does a young person's credit status get set as "best by default" in the beginning, or derived from their parents one?

It gets set based on your credit history, which starts out as nothing or very little so your score is low. However, if you have a bank account and a stable income, and/or a savings account, as well as stable spending habits then those things go towards your credit score. You will not be eligible for large amounts of credit in the first instance, until you've built up some credit history.