I am thinking of this trope of the true heir to the throne being someone who was born/raised in ignorance of his noble status. Maybe because he was illegitimate and raised by the mother or because he had to be given away to protect him from an all too ambitious family.
I am not aware of anything that meets your specific scenario. There have been coups that brought lower class leaders into power, but those really aren't found kings.
But in terms of individuals not raised to be King, you would consider George I of England to be a good example. With the Act of Settlement in 1701, he moved from about 50th in the line of succession to 2nd. Those above him were related to the throne through James II who had abdicated during the Glorious Revolution. He was 41 years old at the time.
http://en.wikipedia.org/wiki/George_I_of_Great_Britain#Accession_in_Great_Britain