That is true. But we don't see books being attached to ages, we see them being attached to cultures (if they had any books, that is). The ones you mentioned all came from fairly close geographic location, and with some of them, such as Krishna and Buddha, or Christ and Muhammad, we see some similarities because they are from specific regions such as India. Should we really be surprised that the Abrahamic god is so often associated with the desert, or that the Nile had even religious significance to the Egyptians?
Most of such people are not "updating," but establishing new laws and new beliefs for their people. Some have "updated" or "added to," and of course many split and branch off, but that most cultures have established their own religious views is nothing remarkable. We even see it happening today with more mainstream scientists expressing belief-in and having experiences a spiritual experience. But their morality is not coming from a god, but rather from ideas that in some way or another could be described as the principle of lesser harm. Of course we can see a "no harm" being practiced by the most devoted of Jains, but in this developing approach to spirituality it is consideration, thought, and conscience that guide moral decisions rather than a fear of offending god.
But, even that is following this pattern of "new and relevant." For the most part, the only thing it has to do with "the old" is pointing out how abhorrent so many parts are, how inaccurate it is overall, and how far along we've come since then. And that itself often only really comes out when having to defend against absurd claims such as how "atheists can't be moral."
