Technically, you can't. In reality, most masters continue to learn, never feeling that they have actually mastered a martial art. The majority of schools say the minimum time is three years to earn a black belt. And that is just the starting point, as there are ten degrees of black belt.
You become a martial arts master by studying a single martial art for many years. It is not something that is easy.
Ninjutsu
No martial art is the best. Only those who can master it can be better then another person. Even karate history comes from Chinese martial art.
A martial art's master may rank up to a 10th degree black belt.
You need to pick a martial art and study it for many years. A true master will study for 30 to 40 years before being considered a master.
If you eat Cheese
Yes, Judo is a martial art that was based on JuJitsu.
Its not a martial art its a training drill.
It is Majest Martial Art.
You can learn a martial art at any age. As long as the individual is willing to work at it, they can learn a martial art. It may take longer to master the physical side of things, but the mental is often easier.
Martial arts is a lifelong endeavor. One never truly masters the art.
all of them