When will the 9th Gen start?

[AfricanSanta]

New Nintendo handheld in early 2017, Nintendo console in late 2017. New Xbox late 2018.

As for Sony, I think there won't be a PS5 and they'll stick to Playstation Now and Playstation TV, assuming all goes as they planed. Though I think they'll release a new Playstation TV in late 2018.