Do younger players continue to develop if they are promoted to the majors? I've noticed a drop in development, but I'm not sure if it is due to promotion or because they are 24-25 years old.

it is due to age, most of the development occurs before 23 or 24. Players do continue to develop in the bigs.

I may be wrong, but I think development is much more closely tied to # of professional years than age. If it was related to age, you'd see college players drafted at age 22 slowing dramatically in their development as early as their 2nd pro season. The pattern of development seems to me to be: pro years 1,2,3: fastest development (obviously affected heavily by playing time and coaching); year 4: development slows. Year 5: development slows a LOT.

Players still develop in the majors...the development is slow because it's usually their 4th or 5th year when they arrive.