About 4.5 billion years based on radio active dating using uranium and thorium isotopes. It could be a little bit older if it formed as a liquid mass, since this state resets nuclear isotope clocks which only date from the onset of a solid phase. The oldest meteorites are about 4.8 billion years old so that is probably the upper limit for the Earth too.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.