In Alabama, the age of consent is 16. So, if you are 16 and have consensual sex with a 20 year old, no law has been broken by the 20 year old. If it were illegal (say if you were 15), then you would be the victim and the older person would be the criminal.
404.993.9172 Law Firm of W. Travis Towe. Mr. Towe is a DUI / criminal defense attorney and practices in several metro-Atlanta counties, including: Fulton, DeKalb, Clayton, Cobb, Gwinnett, Fayette, Henry, Hall, Forsyth, and Douglas Counties. All information provided is based on Georgia law and federal law, where applicable, and is not necessarily directly applicable to any other jurisdictions, states, or districts. This response is not legal advice and does not create an attorney/client relationship. Rather, this response is in the form of legal education and is intended to provide general information. Any specific concerns should be directed to an attorney who is licensed to practice law in your respective jurisdiction.
It depends what you mean by "dating." It is generally legal for any two people to be friends with each other. "Age of consent" laws deal with whether or not it is a criminal act to have sex with someone. Various web sites give the ages of consent for the different states, and you can probably get this information from your school guidance office or health center, or from your personal doctor, or the nearest planned parenthood.
Assuming you are not an emancipated minor, your parents have the right to make legal decision for you until you are 18, and this generally includes setting behavioral expectations and the right to approve or disapprove of your friends and social activities.
If you want to date anyone, I recommend that you and your partner discuss your intentions with your parents, and get a clear understanding of their expectations.