If I remember correctly, they were slaves that were kidnapped in their youth.
I don't believe that is true. Janissaries were non-muslims (primarily christians) given a chance to earn privileges not otherwise available to non-muslims in the Ottoman Empire, and eventually became more privileged than the Muslim portion of the military (which lead to the decay of the Empire and loss of significant amounts of territory, but that was a bit past the middle ages when Janissaries started becoming that influential).
