As early as possible! Personally, I was in extracurricular math tutoring from a very young age (around 9-10 years old), which helped me build essential foundational skills in math and reasoning.
I think of tutoring similarly to being taught at school - you wouldn’t want your child at a bad school nor with a bad tutor, but good school or tutoring at any age can be helpful.
As to what constitutes good - a balance of challenge that will make your child learn more and explore the world (academic and non-academic) but not so much that the child is stressed out and will compare themselves to others too much, and certainly nothing that’s going to interfere with simply growing up and being a kid.
A good example I think is trading a child’s excess screen time on an Xbox or computer with more constructive things like tutoring. Effective tutoring can be incredibly beneficial for anyone. I like to think of it as personalized schooling. The real art is nurturing a child to be naturally interested in academia so that school (and tutoring) can be looked forward to. In some ways I wish I’d had tutoring during school to accelerate my academic pathway and help me to understand that I could excel at subjects I didn’t think I could be good at (mainly English based things), however I was lucky that I went to a great school (Lindisfarne) and was very engaged with learning.
The key is definitely to start good quality tutoring as soon as possible but this needs to be tempered with the need to have the tutoring sessions with a strong component of fun.
It is critical with younger students (9 years and below especially) to have tutoring which is very personalised and a source of fun for the young student and not an onerous task. It needs to be with a tutor that incites delight and curiosity to garner a genuine enthusiasm for learning at a young age.
This “fun” might be captured with something as simple as computer games, stickers or a trip to somewhere which ties into the tutoring - for example if being tutored in maths - an age appropriate trip to show that young student the relevance of what they are doing.