As yet, there is no scientific agreement as to the age of life on Earth. Estimates (based on geological evidence) currently vary by about 200 million years. That's to say between 3.85 billion years ago (Ga), or 3.65 Ga.
To date, the oldest undisputed fossils appear in rocks from 3.2 Ga, but recent carbon dating techniques applied to rocks from Akilia Island, in Greenland, and which checked the ratios of carbon isotopes, found suggestions of life at 3.85 Ga.
Further reading Astrobiology Magazine (NASA), 2006
Also see: Primordial Soup