Tom Marvolo Riddle was born in the last hours of 1926. His first year was 1927, his second year was 1928. Counting on in this way, we discover that his sixteenth year was 1943, which is when these murders occurred:
"In the summer of his sixteenth year, he left the orphanage to which he returned annually and set off to find his Gaunt relatives."
Years later the son, now beginning to call himself Voldemort, sought the Riddle family out and killed them all in revenge (HBP17), and used his father’s death to create a Horcrux (BLC).
"In the summer of his sixteenth year, he left the orphanage to which he returned annually and set off to find his Gaunt relatives."