Complex sentence: When he was eight years old he was sent to Tsarskoe Selo to enter the Alexander Cadet Corps, and three years later he entered the Sea Cadet Corps at St Petersburg, making his first voyage in 1858.
Split into simpler sentences: When he was eight years old, he was sent to Tsarskoe Selo to enter the Alexander Cadet Corps. Three years later, he entered the Sea Cadet Corps at St Petersburg, making his first voyage in 1858.