The range of years from 18 to 21 really does cause a big change in a person. Once she gets past those years, establishes a residence, begins a career, and starts to get a sense of who she is in the world then it could work. The problem with it at age 20 is that if she hooks up with a 45 year old who has already arranged for the basics then she might be tempted to skip doing things for herself. She might miss out on this and have trouble making up for it later on if something happens to the relationship. For a relationship to be a true partnership, both people must be on equal footing.