He should have been around 16 when he killed Tom Riddle senior. In Half Blood Prince chapter "A Sluggish Memory", in the Slug Club gathering memory, it states Tom was already "wearing Marvolo's gold and black ring; he had already killed his father". It also stated all the boys were in their mid-teens. He was definitely 17 at the oldest, as that's the age students graduate from Hogwarts. And he was older than 15, as he was already a prefect (who get appointed in their 5th year, when they are 15).