Rosemary Sutcliff often said that she ‘wrote books for children aged 8 to 88’.
… W.H. Auden wrote that ‘there are good books which are only for adults, because their comprehension presupposes adult experiences, but there are no good books which are only for children’. In this sense, it is natural for children’s books to become adult books if they are any good; since all adults have been children, books for and about children are always potentially for and about adults too. (Hugh Haughton)