All I could dig up was some info from the CUSWE
saying that he was taken from his family from an early age for his Force affinity and was born out of a moderately wealthy family. Hehe, just another of saying he was a middle-class commoner.
As for Yoda, nobody is really sure about him, Lucas has been pretty secret on the green dude. There was Yaddle also though (same species as Yoda), so I suppose they are some really rare species with a rare gift of high Force affinity, something like the Fosh (Vergere) and their special tears.