Orlando is experienced in the play. Although he hated his brother, Oliver, in the past, he saved Oliver when Orlando saw him in danger. It showed that Orlando has changed during the time in the forest. He tried to accept and forgive his brother.
As for Rosalind, she made herself as a Ganymede. Although Rosalind never have any intimacy issue with boy, she did not only learn to love but also teach the other how to love. She finally fell in love with Orlando.
Being experienced is not equal to being mature. Although Orlando and Rosalind are both young, they still experienced something in the Forest of Arden. A person may have different points of view or different reactions on the same thing when this person are in different life stages. For example, a young man may hate the taste of alcohol but may love it when he grows up. Thus, it is not necessary that only the old had the experience in the play.