As soon as he comes forth, his lights begin to act. However their actions are not complete… We see the same below: the newborn baby is active immediately after coming out, but his actions are incomplete and without full intention. Likewise above, the actions of the lights of Zeir Anpin are incomplete without the repairs they are to receive from the supernal lights.
…but undergo a steady process of repair through Imma… For all the repairs to Zeir Anpin come from Imma. The further each light extends, the greater the repair it brings. …until they become complete at the time of complete Maturity. For complete maturity is when the Mental Powers are fully functional, and so it is down below. By the time of maturity man has already reached his full mental powers.