Tafsir of An-Naba' 78:33

(And kawa’ib [full-breasted maidens], atraban [of equal age]):

Kawa’ib is the plural of ka’ib, which refers to a woman whose breasts have begun to swell, becoming rounded with a slight protrusion. This occurs at the age of puberty and the most excellent state of development.

Atraban means they are of the same age, growing up together. This is either a comparison regarding their equality and resemblance to the tara'ib (the ribs of the chest), or it refers to them being upon the turab (the earth/ground) together. In some commentaries, it is stated that all the women of Paradise are sixteen years old, while their men are thirty-three years old.