Is Clint the oldest working filmmaker?

Well, as of today he might be indeed...
Carlos Saura is not too far, his last feature is actually in post-production and he's 89 :-)
It also reminds of Manoel de Oliveira, a portuguese director who died in 2016 at 106 years and directed his last feature at 103 years old, that's badass!

Alejandro Jodorowsky is almost 92 years old, so slightly older, and still makes quality films.

I think Richard Donner wants to make Lethal Weapon 5 and he's 90 now.