Over the past few years something interesting has happened... Jiu-Jitsu has expanded greatly.
Not too long ago Brazilian Jiu-Jitsu was primarliy a martial art practiced by adults. Adults where drawn to it for it's practicallity and function. Unlike many classical martial arts that failed to deliver on the promise of self-defence, Brazilian Jiu-Jitsu proved itself. Practitioners cared more about function and usefulness then following tradition. There was no memorizing patterns and punching air in lines well counting in a different language.
Jiu-Jitsu did for adult martial arts what the karate kid movie had done for the kids market 30 years ago.
It was fun, it was functional, it was safe, and it got people in amazing shape. It also had a depth of knowledge far greater then most other "martial arts". You could not get a black belt in 3-5 years... this was something that took a decade or more for most people.
As the art grew the kids followed the adults. Kids where drawn to it because it's fun. Kids are natural grapplers, wrestling and grappling is something they do. Parents where drawn to it as well, for the practicallity and the safe nature of it. We all want our kids to be safe, and to be able to defend themselves if they ever need to as well as have the confidence to stand up for themselves to bullies. Jiu-Jitsu teaches a self-defence that is ideal for kids because it is about control, not inflicting damage.
In a traditional striking based art practioners are taught to hit back, harder, and to more vital spots. In the end both people are getting hit, and probably hurt, but to hurt the other person more and faster. In Jiu-Jitsu the response to someone trying to hit you is not to hit them back harder, but to take control of them and put yourself in a better position where they can't hit you and you can force them to give up.
So now Jiu-Jitsu is probably one of the few arts that is truly for all ages. Boxing and Kickboxing have some kids, but it's full contact blows limits it for kids who should not be taking shots. Classical styles like karate and Tae Kwon Do have a few adults, but most schools are primarily kid based. Wrestling and Judo tend to taper off for older adults as they can be quite rough on the body with all the throws. But Jiu-Jitsu can be trained safely and effectively regardless of whether you are 3 or 63. SOme practioners have even continued way past that age.