Why exactly?
The physical prime of humans is in their 20s.
Just look sports stars - nearly all of them in their 20s.
Heck, take Messi for example: His absolute peak came in his early 20s (2009-2013) and that peak is still considered the best that football/soccer has ever seen by quite some distance.
Cant use that real world logic in one piece.
Luffy was a normal wimpy kid. Big mom was born a monster, as was oden. And neither of them was the strongest in the verse at his age, or even near their peak at that age. So its crazy that two characters whose starting points were so far passed luffys, were so quickly overcome by him.
Even just a few months ago, luffy had to jump a yc3.
The marines said 68 year old big mom is stronger than the 30 year old big mom from God Valley. That's 38 years of expirience that 19 year old luffy is about to, if he hasnt already, overcome. Despite the fact that 2 years ago luffy was fodder that would get giggle diffed by vice admiral vergo.