What does wives are young mistresses companions for middle age and old nurses means?

This is a quote from Thomas Fuller, an English writer of the 17th century. He was describing the role of wives in different stages of a man's life. He said that wives are young mistresses for men in their youth, companions for men in their middle age, and old nurses for men in their old age. This means that wives are responsible for taking care of their husbands' needs, both physical and emotional, at every stage of life. They are a source of pleasure, companionship, and support.