thirty, forty, fifty
He looks about forty.
There were about forty people. (Key English Test; A2; Polish)
By the time I'm in my forties, I want to be running my own business.
The man was in his early forties and he looked to be a very important man. (First Certificate in English; B2; Basque)
Most of these people were born in the forties.
So, I would like you to consider buying some English comedies, like those Ealing Studios [made] in the late forties and the fifties. (First Certificate in English; B2; Spanish)