Board Thread:Naruto Discussions/@comment-25139119-20150923133911/@comment-25632268-20151005205121

Madez wrote: When the konoha higher ups were debating whether Kakashi should become Hokage (just before the Kage summit arc), heir argument was that Kakashi was too young. Someone else then brought up the point that Minato was even younger. So I find it hard to believe that Hiruzen wasn't older than Minato or even Kakashi at the time he was appointed, because no one made mention of Hiruzen.

Different circumstances. Minato was appointed either at the end of Third Shinobi War or after it, when the current Hokage (Hiruzen) was still alive and active. Tobirama appoointed Hiruzen just before going on suicide mission, so it left Konoha at war and without leader. I doubt anyone worried about Hiruzen's age at that time.