We're never told explicitly when Denethor was born, but it's notable that the account of his father Lenwë leaving the Great Journey and passing down the Vales of Anduin makes no mention of Denethor. This implies that he was born after that event, which took place about 3,700 years before the first rising of the Moon and Sun. The earliest direct mention we have of Denethor is to his leading of a part of the Nandor over the Blue Mountains into Beleriand, dated as Valian Year 1350 (that is, about 1,400 solar years before the Darkening of Valinor). Thus Denethor was at least 1,400 years old when he fought and fell in the First Battle, and he may well have been considerably older.

'Lithe-and-lank' comes from a note to the essay Quendi and Eldar in volume XI of The History of Middle-earth. There the name Denethor is explained as deriving from dene- 'thin and strong, lithe' and thara-, 'tall and slender, lank', with the entire name therefore translated 'lithe-and-lank'.

It should be noted that an earlier and entirely separate derivation also exists. In this tradition, the name Denethor comes from ndani-thārō, 'saviour of the Dani' (the Dani being an old name for the Green-elves).