Purgatory was only officially adopted in the 16th Centry. Please pay attention to the term "officially adopted". The Council of Florence only raised the concept of Purgatory. It did not make it official dogma. The term used in the Catholic encyclopedia is "began apropos" meaning interjection for further comment - to be discussed at the appropriate time. In essence, this was the formation of the doctrine as there was a need by the Catholics to justify their doctrine that had been rejected by the Greek Orthodox as apart of the larger context of the attempt at reunification of the Latin and Greek. The official adoption was done at the Council of Trent. One could argue that the Bull of Pope Leo X issued June 15, 1520 Exsurge Domi, that since the Bull mentions purgatory, that it is "officially adopted" at that time. However, that is also 16th Centry.
As for when it became Christian practice (as in an expectation or duty) the earliest writings that I know of are from Tertullian (c. 160 – c. 220 AD) which is second centry. The Apostles and Clement of Rome lived till 100 AD yet none of them in any of their writtings call for prayers for the dead. The fact that some Christians continued the Jewish practice shows how the coruption of thought continued into Christianity and that corruption was eventually adopted as doctrine by the Catholics.
It is relevant as you said they pray for the dead. They do not technically have anyone in the "dead state" as in Buddhism, as in Hinduism, once you are dead you get reincarnated or attain nirvana. No "place" after death.
I pointed this out with the other errors in your statement.