Terentyev was born in a large Karelian family of nine siblings and spoke Finnish. Although he skied since early age, he started training on a proper equipment only when he joined the Soviet Army in 1944. In 1950 he finished second in the 4×10 km relay at the Soviet Championships, and in 1951 placed third in the individual 18 and 30 km races; he won all these three events in 1954 and was included into the national team.
Terentyev's early life involved a large family of nine siblings and fluency in Finnish, which led him to begin serious skiing training only after joining the Soviet Army in 1944. He achieved success in various races, including placing second in the 4x10km relay at the Soviet Championships in 1950 and third in both the individual 18 and 30 km races in 1951.