What is the age of the earliest traces of life on Earth? In the article Oldest rocks, earliest life, heaviest impacts, and the
Hadean–Archaean transition (Moorbath, 2005), the author states (based on previous referenced research) that the $\ce{^13C}$ depletion previously stated as being a biotracer of the presence of early life in the metamorphosed sedimentary Archaean aged (~3.8 billion year old) rocks found in Isua, Greenland is disputed, by what the author states as being a result of


  serious metamorphic
  and tectonic complications for a biogenic
  interpretation.


Hence, the presence of the traces of very early life at that location (and age) are not confirmed.

Thus, what is the age of the earliest traces of life on Earth?
Fossils are our strongest, and to most people most accessible, evidence of the great age of life on earth. The fossils in the Burgess Shale are clearly the imprints of critters, but are a mere 505 million years old. The oldest cyanobacteria-like fossils known are nearly 3.5 billion years old, among the oldest fossils currently known. There is Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks, but requires knowledge of isotopic chemistry to understand (or dispute.) Life on earth originated before 3.5 billion years ago.