@themick:
First, big difference between 15/16 and 20. Second, Luffy got destroyed by Kaido and laughed at by Big Mom. I highly doubt he is going to be Kaido alone.
In fact I think OP says the opposite because most strong people are a decent amount older than Luffy, Zoro, Sanji.
Yeah, there's a big difference between 15/16 and 20: in MHA, a 15yo hero student is comparatively much older than a 20yo pirate in OP.
Most top heroes in MHA are on their 20's~30's (40's at most) while the strongest people in OP are always at least on their 50's or bordering it.
And let's not forget 15 is Deku's age at the start of the manga. The hero course takes 3 years, so he'll be at least 18 when he graduates (not to mention it might take even longer for him to become the #1 Hero).
But who cares if it happens at 15 or 19? How are 4 years of any relevance to the point? Also, what Luffy beating Kaido by himself or not has to do with anything!? We are aiming at the conclusion of the story, not the current arcs.
The point is: in shonen in general, the protagonist will reach the peak of their world at a much younger age than the top contenders (and with a lot less experience).
Luffy is bound to become Pirate King at 19~20 with less than 3 years of piracy while even the most competent and powerful pirates ever didn't got there after decades of effort.
Naruto managed to surpass the whole ninja world by 15 and only 3 years as a ninja even though the most talented ninjas had been doing it since they were like 7 years old.
Ichigo became the strongest Shinigami at 17 and with like 1 year as a shinigami (since the 2 years of timeskip he didn't had his powers) while the other Shinigami literally trained for centuries, sometimes millennia.
That's the rule in battle shonen. The actual exceptions are those that understand the weight of years of experience as something you can't beat by sheer will and "power of youth".
If anything, out of all those manga MHA is the one that sets the best scenario for a new hero to already find a spot at the top. After all, in MHA being a hero is a regular job, simply the result of doing a course, studying and getting a degree: and with just like any other profession, someone can already excel at it in their early years as a pro and be at the top of their field among others who been at it for many years (see Hawks. Or Mirio, who wasn't even a pro yet and was already regarded as the most likely to take the #1 after All Might instead of any already established professional).