Well lets think about it. During the 2nd World War the Sannin were what? In their late 20's? Early 30's? Considering this, Hiruzen couldn't have been more than 50. Considering this, Hashirama and Tobirama were Hiruzen's teachers so had they not died they'd be about 70-80's at this point.Nagato was like 7 or 8 when his parents died and the fight between Madara and first Hokage was prop about 80 or 90 years ago.