Seventy does not mean old or young. As Bob Dylan says in Floater:
The old men ’round here, sometimes they get
On bad terms with the younger men
But old, young, age don’t carry weight
It doesn’t matter in the end
In the imagination of the Jewish sages of Late Antiquity, 70 means quality, not quantity. It means wisdom and variety.
There are the 70 descendants of Noah, the 70 names of Jerusalem, and the 70 wise men of Jerusalem. Legend speaks of a gathering of 70 translators of the Hebrew Bible into Greek for the first time — hence the Septuagint. Most important are the 70 faces of the Torah.