You could say that youth is contrasted to maturity, for example, in the novel because Anne is the epitome of modern practice while other characters such as Mrs. Smith are still stuck in the old ways.
Anne is not rushing into marriage,. Instead, she is into waiting for her perfect someone, and into feeling herself as a part of a romantic, loving relationship. She had to wait, mind you, for nearly seven years and she had to endure the indifference of CPT Wentworth when she saw him again. Had the character of Mrs. Smith done the same thing, she probably could have learned to know the weaknesses of her husband, and could have avoided getting into the pickle that she got into when he began to squander away the money.
Hence, Mrs. Smith is a representation of the "old ways" when women had to accept a husband only for the sake of their financial benefits and forbade the idea of love. Contrastingly, Anne is a symbol of a new generation of independent thinkers (much like Jane Austen was) that prefers to "hold their horses" and hope for a connection with someone whom they can understand, love, and cherish for many years.