During a visit to charity Child Bereavement UK in London yesterday, William was seen speaking quietly to a 9-year-old girl about the loss of her father.
"You know I lost my mummy when I was very young, too. I was 15 and my brother was 12," he told her, according to the Daily Telegraph.
Lorna Ireland, a parent at the event, said William told her 12-year-old son that as a teenager, he was "very angry and found it very difficult to talk about" Diana's death.