What Steve said.....except.....
I think
that the emmissions regulations dont necessarily change every year, may only be every few years (I think 2003 was the last major one?) so the engine might be able to be more than 3 years old at time of completition so long as it complied with emmission standards at time of chasis purchase.
For example, my engine was from a 2002 AUII but just had it registered 2006