At some point 2 grains of sand stuck together that would eventually become the earth, but is that actually the earth? Before the moon was formed in an impact with another proto-planet, it was not the same size or composition as today, so at that point it was also a proto planet. That impact was the final step to those proto planets becoming the Earth and Moon, so personally I would say the earth is 4.5B years old, the point it became its final mass. (Also, the cores of the 2 proto planets would have combined at that point giving the earth a large core and powerful magnetic field).