If they are written much later than Jesus' death, then they missed one VERY key prophecy that Jesus foretold of. In 70AD, the Romans sacked Jerusalem, and tore down the Temple, which Jesus had prophesied in Matthew, Mark, and Luke. If they had been written much later, one would be very quick to logically ask why this prophesy wasn't included in the text of those three gospels, especially if they had been written so many year later. I have a Schofield Bible, which puts the writing of the Book of Matthew, at or around 50AD, which was only 17 years after Jesus was crucified and rose again. Even the Book of Luke, and the Book of Acts, which were both written by Luke, a traveling companion of Paul, were written in 60AD. You also have to think that these books could have been possibly written before this, and just not published till a later date, due to the conflicts in the region.
Mark is the first of the synoptic gospels, written shortly before or shortly after 70 CE. Matthew and Luke were definitely written after Mark.
John was written between 90-100 CE.