There are mixed historical views on this. The Karmapa is an emination of Avalokiteshvara. In his first lifetime as the Karmapa (before that became his name), he learned from Rechungpa and Gampopa. Gampopa said that the Karmapa (with his name before that) was just pretending to be his student so he could hold the lineage, that he had already had a *certain level of attainment (*i do not remember how this level was named or described)(1). Yet there is also a distinct moment in the lifetime of the first Karmapa that he attained enlightenment, whereupon the Dakinis came down and wove him a Black Hat out of their hair (2).
I hope i did not unnecessarily confuse a clear historical account.
Also, each time a Rinpoche is reincarnated they seek empowerments and transmissions so they can again realize, to "make" the real be real for them. They still have a human body-mind.
You seem to understand they would want or need to do practices, which implies an already-imperfect or potentially corruptible body-mind for them, so it is subject to conditions, thus empowerments and transmissions, or teachings, could be useful. Vajrahridaya's point about puberty is good too, i think. Would like to know the source of that.
AR
(1) A Youtube video
(2) The Life of Gampopa 2nd. Edition, Jampa Mackenzie Stewart