Well, honestly, that will depend on what boards you are hanging out on and what CM based “curriculum” you are looking at using.
I don’t think anyone using SCM here, or similar have a problem with starting with a 6yo.
However – if you are looking at AO or some others, they would mostly advise waiting until age 7 or very near age 7…. (and as someone that started AO with a young 6yo…. I would say if you are doing AO, wait until age 7!)