One reason for the united states’ interest in the open door policy for china was that it had just taken possession of which region, after the spanish-american war?

 The Philippines

Further detail:  The United States had taken possession of the Philippines, as well as Guam and Puerto Rico, in 1898, after winning the Spanish-American War.   So US interest in Asia was heightened.  At the same time, other nations had begun competing for "spheres of influence" in trade access with China.

The Open Door policy was issued by the United States in 1899-1900 as a series of dispatches from the US Secretary of State to other nations that had trading interests in China --  Great Britain, Germany, France, Italy, Japan, and Russia.  The policy reasserted earlier agreements that all countries should have equal access to ports in China, without undue preference to "spheres of influence" for one nation or another.  The United States was seeking to maintain an equal footing with other nations in the access to trade in China.

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.