Complex sentence: He was only eleven years old when he entered as a student in the Imperial Academy of Arts, where he studied under the guidance of his father, Andrei Ivanovich Ivanov, a professor of painting.
Split into simpler sentences: He was only eleven years old when he entered as a student in the Imperial Academy of Arts. He studied at the Academy under the guidance of his father, Andrei Ivanovich Ivanov, a professor of painting.