Depends on the type of personality. If she was shy, quiet, a bit nerdy I would understand and have no problem. If she was outgoing I would be suspicious that she did have some kind of relationships. 20 is still really young and highschool and your teen years can fly by before you know it, so I think it is really easy to get to that age without dating much or at all. 25 and up would be more to think about.