The fifth mizukage and the sixth hokage doesn't look old at all. And I don't believe that Ōnoki had died. Why are the kages all different?
Why did most kages change in the Boruto movie?
Because they felt it's time to get retirement and they also found there replacement. Hiruzen also got retired when he found his replacement in Minato and become Hokage again when he died.
Ōnoki always complain about his back pain and after the tiring Fourth Shinobi World War it seems like a proper time to get retirement.
Naruto have a dream to become Hokage from start and Kakashi might have noticed the change in Naruto to be worthy of it so he retired.
Gaara is quite young and capable and still being Kazekage.
Mei Terumī always complained about having scarcity of good man in her life and Chōjūrō being the last of Seven Ninja Swordsmen of the Mist have proved his worth in Fourth Shinobi World War. So Mei might have retired to concentrate on her personal life and also found a good replacement for her.
Fourth Raikage (A) was ofcourse old enough and he even lost a hand just before Fourth Shinobi World War, seems like a proper time for retirement. And Darui proved his worth in Fourth Shinobi World War too.
So mostly it seems Kage just gave there seats too younger generation who proved there worth in Fourth Shinobi World War.