how old does a child or adult have to be, in order for his or hers passport to be good for 10 years and not 5?

Answer 1

According to the US Code (Title 22, Chapter 4, Section 217(a):
A US passport shall be valid for a period of ten years from the date of issue, except that the Secretary of State may limit the validity of a passport to a period of less than ten years in an individual case or on a general basis pursuant to regulation.

Note that for other countries, the duration a passport is valid is not necessarily 10 years. (For example, Canadian passports need to be renewed every 5).

Answer 2

it is 16 years old.