These examples are taken from an English grammar book:
I voted for the Greens.
In what temperature do you wash the whites?
The blacks of Los Angeles rioted last week.
Five-year-olds don’t go to school in Finland.
My question is: Why don't you use the definitive article in this last example? Why not "the five-year-olds"?
Articles are a difficult part of English.
I have consulted my Practical English Usage book (Michael Swan), and the best explanaion I can find is:
"In English, when we are talking about people or things in general, we do not use the with uncountable or plural nouns"
This makes sense, as if you look at your other examples, they all refer to something particular.
I voted for the Greens. (Some kind of particular group or party, etc.)
In what temperature do you wash the whites? (the white clothes/items in particular.)
The blacks of Los Angeles rioted last week. ("Blacks" referring to a particular ethnic group of L.A.)
Also, my book Learner English (Swan & Bernard Smith) states:
"The definite article occurs in Scandinavian languages before uncountable and plural nouns used in a general sense. In English it is normal to use no article in these cases."
I'm not a teacher.