He just passed away i think, prolly not from old age though cause when Kushina was visiting his wife he was dead already maybe he was weakened from his battle with Madara and just died a couple of years later
Hashirama met Kushina with Mito when she emigrated to the Fire Country..He was old and wizened at that time , so I reckon he died because of old age?
There's a 99% chance he died naturally , owing to how strong he is praised to be