We saw when Tobirama died. He is brought back in the same state as he died, cause he died "young".
Hashirama died before Tobirama, and as they are brothers with a good guess of 5 years between, it makes perfect sense....
with a little bit time and modification Oro surely could have made hiruzen younger,
but he is neither senju nor uchiha and i guess kishi didnt even thinm about this possibility.
he just got revived because he also was in the shinigami belly and belings to the hokages.