In GENERAL here on Earth, the longer an animal takes to mature, the smarter they are. So that tracks.
Why? Because that's not how it works with humans?
To quote Larry Niven: "The thing about aliens is: they're alien!"
For me, the "ridiculous" aspect is more just that the Star Wars universe is overly reliant upon some pretty extraordinary coinkidinks. I mean, when Djinn Djaren "adopts" Grogu, he (Grogu) is approximately 50 years old and he knows almost
nothing. Then, within seemingly days or weeks, he is well into this super accelerated (or excellerated) learning phase--almost like he's some sort of "sleeper" agent or something, and he already
knew this stuff, it's more that Djarin somehow triggered this active phase.
Were this an isolated incident, it wouldn't be such a big deal. But Star Wars fully embraces
deus ex machina in storytelling. Still, it could have been interesting had Djarin adopted Grogu and spent several years with him before he ever did anything extraordinary. A bit boring, maybe, but think of the payoff--say 10 or 12 seasons of
The Mandalorian wherein Grogu just sits around in that little hovercrib and does nothing, and then, all of a sudden, he is awakened and completes his Jedi training within days, or even hours,
and he becomes a Mandalorian at the same time!
Of course, I am neglecting to consider that Grogu is also traumatized--he witnessed all of his people being slaughtered or something like that (it's really not all that clear). Maybe he just needed to be in the company of a genuine, caring individual in order to be awakened.