Why was the Third Hokage not revived at his prime? He could be much stronger because it was believed that he already surpassed the previous Kage of Konoha. At that point, he knew all the techniques in Konoha.
There are a couple of possible reasons for this, though as far as I know, it hasn't been explained in the manga.
The first possible reason is that Sasuke hadn't mastered it well enough to bring him back at his full power. As is stated on page 10 of chapter 620, if the summoner doesn't have full mastery of the summoning, the reincarnated individual might not be brought back with the full power they had in life.
Also, according to here:
As a downside, the reincarnated seem to retain any permanent body damage and physical limitations they received during their lifetime.
This could include aging. The First and Second Hokage also seem to appear the same way as they did whenever they were shown in flashbacks.
All of the characters have been reanimated in their most iconic form, we've always known Third Hokage as an old man.
As you can see, all the revived character in naruto was revived in the form before they died. The third hokage died at an old age that's why he was not revived at his prime.