With minecraft 1.3 emeralds were added. Is it possible to find emeralds in a world that was created with an older minecraft version? If yes, does this take effect only on new generated chunks?
Yes and yes, the new chunks will be generated with the 1.3 code and therefore contain emeralds. Since this update has a new generation algorithm you may also need to convert your file (like when going from beta to 1.0) and biomes may have shifted.
It depends on what you mean. Chunks generated in pre 1.3 will not have emeralds in them. However, if you're in a world that was created pre 1.3, as long as you go to the edge of the previously generated part of the world, newly generated chunks will have emeralds in them.
As @BlaXpirit points out, trading is better if you can find a village.