Some famous players learned at 4 or so, for Carlsen it was 8, for me it was 11.
My daughter is 6. I'd love to teach her chess. She loves playing games, but she hates the fact that someone has to lose (if a player is behind in a multiplayer game, she'll help that player instead of trying to win). She also hates the concept of a fixed set of rules that daddy knows and that have to be abided by... so she's invented her own "chess" (take a board and two sets of pieces, put all the white ones on white squares, all the black ones on black squares, and that's it). She also likes to take my Yusupov books and put the pieces on the board just like they are in some diagram, but is completely uninterested in the purpose of those diagrams. This is wonderful, she has her own mind.
It's completely impossible to actually teach her chess at this age though. So the answer to the question can only be "it depends entirely on the child". Whenever the kid becomes interested in learning chess is the best age.
(that said, if they taught the rules of chess at school right now, she'd probably soak them up. But what would that accomplish?)