Why are 16 year old counted in the Civilian Labor Force?

They are not counted as part of the labor force unless they enter the labor force. They are considered to be part of the eligible labor force because 16 is the age you can legally be employed because of child labor laws. That is not to say someone under 16 can't work. it just means those under 16 are severly restricted in what they can do such as operate certain types of machinary and such.