Smaug first appeared in TA 2770 as a mature Fire-drake. I'm not sure if there are canonical sources which say his age or maturity, but he must have been quite large and quite old at the time in order to destroy the city of Dale and capture Erebor.
Since it is safe to assume that Smaug would have been attacking towns and stealing all of the gold and jewels he could get his claws on for most of his life, then he must have amassed a huge fortune prior to taking residence at Erebor. How did he transport his existing horde to Erebor? It seems unlikely that he would have simply abandoned all of his treasure that he had previously accumulated.
EDIT: It has been established that Smaug was not a mature drake at the time of his appearance. However, he may have already had a small horde (compared to Erebor).