Considering Voyager instituted the insanity that Borg don't reproduce, only assimilate, I'd hope they don't age. Otherwise their attrition would be something fierce, and they'd have to continually expand (or keep unassimilated planets in reserve) just to maintain numbers.
We've known that they assimilate babies since nearly the beginning in TNG.
But how early is to early in fetal development to assimilate a baby?
Is it comparable to the current abortion debate?
They can't assimilate a fetus earlier than we think you shouldn't a abort a human being.