3. Obito Uchiha
Obito Uchiha was a dear friend of Kakashi Hatake, and was pretty fond of Rin. When Obito saw Rin dying in front of his very eyes from Chidori blade stuck into her heart by Kakashi, his Mangekyou Sharingan was awakened. At this stage, he was just 14 – 15 years old. His Mangekyou Sharingan granted him the ability to teleport himself, and other things to Kamui dimension. It also made him intangible when he wanted.
2. Itachi Uchiha
Itachi Uchiha was considered to be a genius, from childhood. His best friend was Shisui Uchiha, and when Itachi saw him taking his own life, he was able to awaken his Mangekyou Sharingan. Itachi had already awakened his Mangekyou Sharingan when he became an ANBU captain at 13. So, he must have been about 12 years old at the time of awakening his Mangekyou Sharingan. He could produce Amaterasu flames with one eye, and with other cast a Tsukuyomi with the other.