Young people have black----(hair) and old people have white-

Hair is both singular and plural.The hair on your head refers to all the individual hairs.头发可以是单数和复数,用hair就可以代表你头发上所有的头发~
The plural for countable hair is hairs.One hair,two hairs...hair头发本身是有可数的~可以加s
But a collection of hair is hair.不过以整体来说习惯用hair
一般是通用(比如我们说脸上的毛可以facial hair/hairs)
这里用hair one