Dojos... will let a person try it out for free at first.
Most dojos will start a child at 5 years old.
My daughter takes Karate. She LOVES it. She's been taking it for a few years already.
The youngest ones there are 5 years old.
Kids of ALL energy levels. There are even ADHD kids there. And it is fine.
She did try Tae Kwon Do, but did not like it.
Martial arts, is NOT ABOUT "aggressiveness." It is about, discipline.
Control. Gauging things and reactions and situations... per yourself and, others and your body/coordination etc.
And sure, it is a physical activity.
BUT, it is a lesson. In a person's whole being.
And if he is out of hand, the Sensei, WILL speak to him and correct him.
My daughter's teachers/dojo is very traditional.
If a kid acts up, they get reprimanded.
So as a parent, know, that.
And as a parent, you can ALSO tell the Sensei, about your concerns about your child. The Sensei teachers at my daughter's dojo, KNOW that some of the kids are ADHD or Aspergers etc. You as a parent, TELL the Sensei that, so that they are aware of your child's, concerns.
That is being, responsible about your child, to the Teacher.
A martial arts class, would be beneficial.
But keep in mind, that at least at my daughter's Dojo... they do NOT "coddle" a child. So as a parent, know that.
And "show offs" are not, tolerated.
And at my daughter's Dojo, some of the parents ALSO take the class, with their child! There are all ages there, kids and adults. Even my daughter's Sensei, his own Sensei will be there too sometimes... and he is at least in his 80's. A top level Karate Sensei, world wide.
At my daughter's Dojo, classes are not expensive. It is not a "money maker" for the Sensei at all. They do it, because they want to help kids, not to make money. And these Teachers, are all, well known in the community and are all high ranked black belts, some being Police officers as well. And they have been teaching for decades. Their Alumni, even if now grown up, will still come by and refresh their skills and/or help to teach the underlings as volunteers.