How did columbus decide that he could reach the indies by sailing west?

Because the map in his time was wrong, China was thought to be farther east than it was, the Americas were not on the maps the combines Atlantic and Pacific was thought to be much narrower than they are.

depends on the age of the teen, many believe by the age of 16 a person should know the strong difference between right and wrong along with the difference between risky and immediate death. swimming with sharks might be dangerous but swimming with sharks while bloody meat is attached to you is just insane. even more if you have no idea what you're doing. however, no matter the age, as long as the person is skilled enough and has the mental and physical capacity to control their actions doing whatever sport they put their mind to is all that matters.