Board Thread:Naruto Discussions/@comment-25139119-20150923133911/@comment-3965545-20150929172705

Hashirama died 50 years prior to 16 ANB(After Naruto's Birth) and chapter 303 confirms so. So in 34 BNB Tobirama became Hokage. The Sannin's were born in 38 BNB so they were definitely alive when Hiruzen became Hokage and in fact he was actually Hokage when he was the Sannin's Sensei.