1.sixteen-year-old是个形容词短语,通常作前置定语,其中注意它既然是个形容词,当中的year就不能加s变复数.如:She is a sixteen-year-old girl（她是个十六岁女孩儿）year单复数是常考的,切记.
2.sixteen years old“十六岁”.这是表达某人多大年龄用的固定结构啊,如:Tom is sixteen years old(汤姆十六岁了)
sixteen year old中的year应该变复数,因为sixteen是复数.至今,我还没见过sixteen year olds这种用法,再说old是个形容词,不能加s的.