there is no way shanks could have learnt more than fluffy at age 19. fluffy and shanks are currently equal so it makes no sense for a no. mc to be that strong at age 19 when it is not relevant to the story
You made those Rules, not Oda.
Luffy reached this Level thanks to his DF & Haki, not just Haki Alone
There is no such thing as Relevant to the Story, it's just what Oda wants to Draw,
If He wants to make any Character that Strong, it's his choice, i don't make the rules
And Characters don't magically jump Levels every Single Year, you have a Potential, and each Character will reach it at different Rate, after that you stop growing
Shanks was already this Strong eons ago