How old is the Old Testament and, like the New Testament, are there multiple versions (i.e. - according to Matthew, Mark, Luke & John)?
Really the best way to get your head around this is to read it through, then ask more questions about what it all means.
The Old Testament (or Old Covenant) dates was written over a period of at least 1,000 years, beginning possibly around 1500 B.C. Genesis may be the compilation of writings dating back much further, and that could increase the age to the time of Abraham (2000 B.C.), or even Noah (2500 B.C.) or even Adam (4,000 B.C.).
The last Old Testament writings occurred close to 500 B.C. (This is excluding aprocryphal or inter-testamental works).
Are there multiple versions? No. I would say, though, that some retelling of stories does occur in 1 & 2 Chronicles of events that are recorded in 1 & 2 Samuel and 1 & 2 Kings. Deuteronomy is a summary of the exodus of the Israelites throughout Exodus, Leviticus and Numbers. Psalm 78 retells a brief history of the Jewish people as well. These "retellings" are all complementary and not contradictory.
Regarding Matthew, Mark, Luke and John, it's necessary to mention that these are not four contradictory version, but four complementary accounts.
We need to ask the question of what it would be like if they were all identical. In that case, there would be only one account and not four, so four identical accounts is of no value. Four accounts that tell contradictory accounts of the life of Jesus (i.e. one says He was stoned, another says He was beaten to death, a third says He was crucified, and the fourth says He never died) would render at least three of the accounts false. However, four complementary accounts give corroboratory evidence to the events recorded therein.
Again if four eyewitnesses in a court of law say precisely the same thing and remember everything each other does, we would conclude they got together to get their story straight. If their stories recount contradictory facts, then they are lying. Yet, if they include different details from each other, but all of their stories fit together, then this is strong evidence that the story is true.
So, the Old Testament is like this in its retelling, and the New Testament is like that as well.
First of all we must understand that there are no answer concerning the dating of OT texts that everybody agrees upon.
Second, there is a difference between the time of the events and the time of writing down those events in their final form. Before the invention of the printing press societies relied more on oral traditions, which science has proven can convey messages verbatim across several generations with equal precision as hand-written manuscripts.
Third, the texts in themselves are quite silent about their authors. It is never even stated that Moses wrote the complete texts of the Pentateuch, even if that tradition is strong.
I seriously doubt that Henok or Noah spoke biblical Hebrew or that even Abraham did. Thus the exact words that speak about the occurrences in Genesis did probably not take shape until after the invention of the oldest form of the Hebrew alphabet.
Weighing all things together my estimation is that the earliest parts of the Bible probably were put down during the time of Moses, but most of it were written after the exile, probably by Esra and a few learned friends of his.
However, there is not a single piece of my faith that is shaken if evidence would appear that alters my tentative dating of the texts.
As for versions, some events are told twice or even more times. The creation is described twice in the first 2 chapters of Genesis and then there are some texts describing creation in the Psalms. Deuteronomy = "the second (deuter) law (nomia)" retells the events in the desert. Chronicles retells some of the events from Kings. And yet a few events are depicted again in the Prophets.
However, the Christ event is unique and of far greater importance than anything told in the OT. It is thus only natural that we get a description from more angles.