Whats the youngest age to put a child in martial arts?

As young as four years old you say. I have to say this. When I teach martial arts, notice that most of students below the age of 10 years old do not comprehend the style in which they study. But that does not mean that children are not mature enough to learn martial arts. There is an 8 year ild in martial arts class who is really good. His technique is sublime for his rank. Not only is his technique good, but he also understands the moves in which he understands

It will only work if the CHILD wants to train in a martial art. Not if YOU want your child to train in a martial art. Wait a little bit, and see if they show an interest in martial arts. I was in grade 5, and am in grade 10 now and still highly interested in it. I see little kids( and bigger kids) at my club all the time who have no clue what they are doing. It's a shame and a waste of space on the dojo floor. Talk to your child about it. Don't make your child become one of *those* kids, no offense.

"Youngest" and "most reasonable" are very different things. I've seen 18-month olds in classes designed to get them walking, exercising, socializing (read: being babysat). However, most places will take a potty-trained child - around 4 - and do actual martial arts stuff. Most reasonable? I think when a child is capable of holding an intelligent conversation with an adult, which to me is a reasonable indicator of reason and ability to comprehend difficult subjects. This means from about 5th grade on. They'll be on-track for earning a legitimate black belt by the time they graduate high school. Too often, though, children start earlier and earn their black belts before getting into high school, and to me this is way too young. From a competitive point of view, this is extremely common, but from a self-defense perspective, this is reckless.

Listen to pugpaws. It should not be baby sitting.

I'm a triple black belt. I have coached students as young as the second trimester in the womb.

any kid less than 13 in my opinion is a waste of money

It really depends on the child. Some children are more mature mentally at age 6 than others are at age 10. Usually around age 5 is about the youngest I'd ever suggest anyone consider putting their child into a martial arts class, since that's really when the average child can start to understand structure and follow direction (in the U.S. public education system, children start Kindergarten at age 5, just to help put things into a bit of perspective). Personally, if I were to ever run my own martial arts school, I wouldn't want to teach anyone below about age 12, though that's the way I tend to be personally (admittedly, I'm just not too enthused about teaching younger children *anything* in a formal setting), rather than any belief about at what age people should start martial arts.

Honestly kids shouldn't start martial arts before the age of twelve. Twelve year olds can learn everything and understand some of the higher more dangerous aspects of the art they are learning. I know this sounds harsh and traditionalistic, but younger than twelve won't be able to learn everything and may never get a black belt depending on the system.

I agree with tyvern, young does have its benefits, but there is a point where they really cant retain it.