I don't mean the approximate time when a certain book in the bible was probably written but the age of the oldest copy of a book in the new testament, or other corroborating literature.
(Since the question was changed as I posted my answer, the original is preserved below)
A bit under 2000 years.
The question of "proven age" is not really answerable to a very high precision, because it is difficult to say when exactly Christianity became a distinct religion separate from Judaism. Apart from the veil of history, you'd also have to draw a line between offshoot religions and mere sects. And there doesn't seem to be any real consensus on where that line should be.
So, the best we can do is come up with a range of years when Christianity formed. The time frame points to some time in the 1st Century A.D.
26 (29) A.D. / 1,988 (1,991) Years Old: We can establish the absolute upper limit to Christianity's age as the start of the Ministry of Jesus. Obviously, there could be no Christianity at least until Jesus introduced the seeds for that religion. The start of Jesus' ministry can be dated to 26 or 29 A.D.; during this time, it would have been at most a sect of Judaism. By the loosest possible interpretation, therefore, Christianity would be 1988 years old.
30 (33) A.D. / 1,984 (1987) Years Old: It is far more convincing to say Christianity began after Jesus had been crucified. The earliest of this position dates Christianity to the Great Commission of Christianity after Jesus died. In biblical canon, after "resurrecting", Jesus told his apostles to spread the gospel to all corners of the world - and this became a central tenet of the Christian faith. Of course, the historical evidence for this event is sketchy at best.
50 A.D. / 1,964 Years Old: More convincingly than the previous two upper limits, we can dates Christianity from the Apostolic Conference in Jerusalem, 50 AD. At that council, the nascent Christian faith ceased requiring circumcision - a noted departure from Judaism. Although Christianity was still seen as a Judaism sect before and after, this doctrinal difference could be interpreted as the start of Christianity as a separate religion.
70 A.D. / 1,944 Years Old: This is the traditional date, and tracks Christianity's separation from Judaism to the destruction of the Second Temple. Up until this point, the Classical World at large (including the Roman government, Jews, and the proto-Christians themselves) still considered Christianity to be a sect of Judaism. In the aftermath of the Second Temple Period's end, however, Rome instituted the Jewish Tax. This led to the early Christians began petitioning Rome for legal recognition as a separate religion from Judaism. This is a persuasive marker for the start of Christianity as distinct from Judaism, although the two remained interlocked and linked for centuries to come.
96 A.D. / 1,918 Years Old: the Roman Emperor Nerva exempted Christians from the Jewish Tax.
The earliest confirmed date of written evidence for Christianity is probably the book of 1 Thessalonians, which is dated around 50 AD. Wikipedia has a list of the various books of the new testament (the portion of the Christian Bible which is specific to Christianity), and their dates of authorship.