Edward was changed into a vampire in 1918 at the eternal years of 17 (he was born in 1901)
He was found by Carlisle at the hospital dying of the influenza and Edward's mother (Elizabeth Masen) told Carlisle to do whatever he could for saving Edward of dying, so Carlisle changed him into a vampire knowing that was the unique form to save him.
1918, he was dying of Spanish Influenza. His real mother did not survive but had told Dr. Carlisle that he had to save her son (Edward) no matter what it took. Carlisle did as she had requested and had turned him just before he had died completely, then Carlisle's wife Esme soon followed with the transformation.
