The source of the information is practically irrelevant, as any decent history of the Japanes warrior arts will verify these ages (give or take a year, depending on the source). The interesting question is why, although that should be fairly self evident. The Ninja began training at a much younger age, generally speaking, than the Samurai. They were often as young as 6-8 years old when they began their training, as opposed to the Samurai, who typically began their training after the age of 20. One reason for this was that the Samurai were usually chosen from the best of existing soldiers, while the Ninja chose trainees who were not already soldiers loyal to the government, often choosing to train from childhood. Another main reason for the age discrepancy is that the Ninja primarily operated under "black ops" situations, with a goal not to be detected at all throughout the mission, while the Samurai were straight-forward soldiers and bodyguards for the Emporer, more apt to be killed because they were directly engaged more often.

Average Age of Death of Ninjas? What kind of question is that?

or perhaps it was one of thoe weird quuirks of fate where they all fell through a wormhole in time, and died before they had actually started training, thereby causing their trainers a lot of annoyance and even more embarassment.

however, thats just a theory. he might have had the ages reversed.

Actually it's not incorrect, the calendars are different. The Japanese used a calendar system which had more days in the year than the Gregorian, in which it's start date (year one) was from the beginning of the rule of the first Emperor. Therefore while it is accurate that a samurai could not begin training until his 20th year, that is based on the Japanese calendar. His age based on the Gregorian calendar (which you are using) would be 17.

The average ages of death were based on the Gregorian calendar, as it is the calendar system most commonly used today.