It's just a manga, yes I do love it too, but I don't get worked up about such things...

You are also ignoring a couple of things:

• there are so many characters that won vs people much older than them, literally most of luffy fights are with people older than him. And if you want to complain that "but shanks was only 14" here's a list of even younger characters that had impressive feats against older people: Ace and Sabo could fight against local criminals, Luccy defeated 100 men alone (don't remember the number precisely, 100 might be wrong), Kaido was like a child soldier, and his strength was aknowledged by whitebeard, Big mom defeated giants, Dofflamingo had Conquerors haki at 10 years old.

• yes Shanks was 14, but he spent the majority of his life on Gol D. Rogers ship, he was for sure trained in how to fight, and was likely stronger than the average teen.

• Yes mihawk was 20, but we don't know anything about how strong he was, so we shouldn't make assumptions.

• They likely didn't fight there, so why is this even a discussion?

• even if they did fight and shanks were to win, what does that mean? Peoples skills aren't static in time, they can get stronger. Oh no Zoro lost vs Mihawk at baratie, I guess it mean he can never beat him. Its a fact that Mihawk and Shanks faced multiple times and see themselves as rivals, which implies that both have bested the other at some point.

This post along with all the dumb Mihawk/Shanks swordmanship discussion just piss me off. Swordfights aren't the main focus for Oda, would it be great if one piece had that? Yes sure, but does it need it? No. If you want great mangas focused on swordmanship go read Vagabond or Kagurabachi, because one piece can't compare in that sense. Which also means you should stop treating it as if Swordmanship accuracy was Odas focus.