What are the health benefits of doing ninjitsu or any other martial art?

Several things: The martial arts can help a person develop strength, flexibility, coordination; it can also help with stress and confidence. These should be weighed against potenital risk for injury. Check with your primary doctor before starting, especially if you have any chronic medical conditions.

Many!: Martial arts can help develop your balance as well as benefit your aerobic capacity, flexibility, and strength. There are many styles of martial arts, and as with any class, you can have a good or bad instructor. It can be the "best" subject, but if your instructor is bad, then your benefits will be less. I would visit several schools or ask around before choosing a school based on it's style.