Age Appropriate Training

Here's my take on what type of martial arts training is appropriate for people at different ages...

For children, your best bet is some form of grappling, because should they lose their temper, be guilty of bullying, or have to use their skills on another child in self defense, at least they are wrestling around with the other child instead of physically beating the other child. Regardless of what the child goes on to study as they grow older, the grappling of their childhood will always be helpful to them. My first pick would be BJJ - it has all the bells and whistles kids like in martial arts, and the techniques they learn are immediately useful even to beginners:

My second choice would be wrestling, though this is usually a less pleasant experience for the child. If neither of those are available, Judo or Sambo is a viable 3rd option. I would avoid pseudo-grappling styles that don't include any actual sparring.

In childhood and early adolescence, cardio training in general is especially important, as it is the main exercise factor that contributes to healthy body growth at this age. BJJ is a very cardio intensive work out, and most BJJ coaches encourage their students to do cardio training (if not make them do additional cardio training in class.)

Once a youth is old enough to have a driver's license, and have friends who drive cars, they can potentially get into a wider variety of self defense situations. While grappling is the best way to handle an opponent one-on-one, striking training (like boxing or Muay Thai) includes skills not focused on heavily in grappling that are very handy if a strangers attack: managing distance, using footwork to avoid attacks, landing blows to the jaw or legs that can significantly slow down or stop an attacker. My first pick here would be Muay Thai:

Other types of traditional kick boxing with low kicks like Sanda, Savate or Knock-Down Karate (like Enshin or Kyokushin) would be my second choice, though they lack the extended clinch fighting of Muay Thai. Boxing would be third viable option if the other choices were not available. I would avoid any self defense style that didn't include any actual sparring, or which did stop-and-go "point fighting."

In late adolescence and young adulthood in addition to cardio, strength training is very important to promote good body growth. This builds a healthier foundation for the rest of their life. A good traditional kickboxing style with low kicks is itself a challenging strength training exercise, and all instructors have their students do strength training as part of their striking training, and many instructors also encourage their students to do additional strength training outside of class.

Adults interested in martial arts should peruse a well rounded martial arts education if they don't already have one. MMA would be my first choice here, because it will help you clear up any misconceptions you might have about your own ability to defend yourself:

If MMA isn't available, than any of the above mentioned martial arts would make a decent 2nd choice. Adults need to build flexibility in addition to cardio and strength training, in order to promote longevity and reduce injury. By this I am not talking about "warm up stretches" I mean going for serious gains in flexibility. Training to do both striking and grappling together as in MMA is a good flexibility exercise in and of itself, and most instructors have stretching exercises as part of their curriculum, as well as encourage students to pursue flexibility exercises outside of class.

There are many interesting esoteric martial arts out there like weapon martial arts, self-defense styles and Tai Chi. But in order to utilize those advanced techniques you need a foundation of solid basics that includes plenty of sparring first!