Complex sentence: He served in the Red Army from October 1924--at age 14 he entered in the Kharkov Military prep school and in 1926 he was sent to study at the Military Cavalry School, graduating in 1929.
Split into simpler sentences: He served in the Red Army from October 1924. At the age of 14, he entered in the Kharkov Military prep school and in 1926 he was sent to study at the Military Cavalry School, graduating in 1929.