Well since their souls were removed from the death god, then it is plausible that it is because that was the condition Hiruzen was put into the death god. Hashi and Tobi were revived in their true prime because they were conventionally revived using edo tensei and then placed in the belly of the death god.
Minato and Hiruzen are different because they both sealed themselves into the belly of the death god, THEN were revived as edos. Therefore, both are not in their conventional primes but rather as they were when they were taken into the belly of the death god.
Edit for clarity: edo revives shinobi at their prime ( Nagato, Madara, Hashirama, all shinobi who died and came back without deficincies that they had at their death )
edo removed from death god revives shinobi the way they were when sealed into death god ( hiruzen is old, and minato has kurama chakra)