I think that's the genius behind making Grogu the species he is. Due to their long lifespan Grogu can remain a child even 5 seasons in.

I doubt we are going to see any huge growth spurt. We know he can live up up 900 years old and he's currently 50. If Grogu's species matures mentally like a human but just slower. At 250 years he'll be mentally equal to a 25 year old human. Although unlike a human baby, Grogu has learned how to fully walk on his own before being able to speak a single word. Human babies tend to speak simple words before attempting their first steps.

Maybe they mature similar to us, just in a slightly different order. The only reason I even compare their aging process to a human is at 900, Yoda was pretty much like a really healthy and active 100 year old human. If I'm right Mando will petty much be dead before Grogu is in the first grade.