I'm not sure, but everything seems to indicate that he died of old age. He knew Tsunade when she was younger and his wife was able to mentor Kushina. These both seem to suggest that he didn't die until quite some time after his fight with Madara.
It is true that normally, Edos are brought back in the condition they were when they died. However, there might be unique ways of bringing back people like Hashirama and Madara. We know that Madara died of old age in the cave with Obito and the Hashirama wall, and yet he was brought back in his prime. This could be the same with Hashirama.