At what age should a kid start a martial art?
It depends on the kid, the teacher, and the martial art.
Is the kid interested? Is he or she old enough to understand what’s going on? Can he or she focus?
What about the teacher? Is the teacher skilled at teaching kids that are about as old as yours?
What about the art? Some arts are inherently fun for kids. TKD lets kids bounce around and kick stuff. Judo lets kids roll around. Taijiquan and many Chinese styles require long periods of dull stance holding and require that a brain able to internalize complex principles and visual concepts. And some arts advocate very aggressive, violent mindset
I find that kids can learn something sensible from the point where they have the emotional maturity to work through something difficult without giving up. From the sample set I have been involved in teaching, the youngest I see that is around six years old, and for some it is eight or nine.
Martial arts are a very different thing for younger children than for teens and adults. But don’t discount what they learn early. My twelve-year-old son has a technical understanding of Taekwondo techniques that is very impressive, though he is yet to gain any real strength which will significantly change how he fights.
Saying which, I doubt that starting much before nine has any material impact on the likelihood of becoming a competitive martial artist, as long as other sports are being played.