And does your lvl 1 wizened old crone have access to level 5 spells? Can your teenage halfling urchin bypass a DC35 stealth or lockpicking check on anything but a natural 20? Does the teenage halfing urchin have vast underworld connections and a mountain of favors owed to him by criminal barons? How high is your wizened old hags DC checks? Tell me what is your Wisdom value and the urchins Dexterity value?
These characters still fall under the exact rules and guidelines I am talking about. A wizened old crone is not the same as starting a level 1 character and in your backstory you have read thousands of spelltomes and incribed thousands of scrolls or you are the headmaster of the academy of magic.
I'm not even sure that you know where you are going with this now.
Those examples you use do not, in any way shape or form, restrict a backstory. You could be the head of a wizard cabal and still be Level 1. That doesn't mean you have to be able to cast
Wish. Being an experience rogue does not mean that you can open locks of a certain DC. Mechanics do not equal backstory.