Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "I had recited the Muhkam, meaning the Mufassal (the last portion of the Qur'an), in the time of the Messenger of Allah (peace and blessings be upon him)," and he was twelve years old at that time.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10530