Yes, as a 21 year old or any twenty, thirty, fourty something...you will be referred to either Miss, or Ms. your preference on how you wished to be percieved. In any case of marriage, deceased or not, you then by title of law are referred to as Mrs. By order of divorce, you then are referred back into Miss, or Ms. according to your decretions. Ms. is commonly used when another person is not aware of your marital status or if you wish not to acknowledged as a 'single person".