What doing you mean as dating? Being friends and hanging sure no problem except 18 and 12 years enjoy different things typically.
My response to your question is a generic response and should not be construed as controlling to your case. I can not effectively advise about your case without knowing all the facts. Additionally, my response does not create an attorney-client relationship. You can contact my office to schedule an appointment if you would like to have me represent you.
If this is something that you are seriously considering, I belive that you may need to speak with a psychologist. It is difficult for me to understand how someone that is 18 has similar interests to that of a 12 or 13 year old. If the interst is sexual, that is even more disturbing as it would indicate more serious issues. If this is a serious question, you need to speak with a Dr and not an attorney.
I am going to be totally candid here, because you don't need legal advise, you need some common sense.
Just picture your life for the next 10 years plus as a registered sex offender. I am not sure of the specific laws in your state, but you will most likely have to register as a sex offender. Your name and offense will be listed on many websites, including links to maps of your place of residence. Your family, friends and neighbors will probably all find out you are a sex offender. Your employment opportunities will be severely limited. Your marriage hopes may be dashed. If you have children, their friends will not be allowed in your home. You will have to check in with law enforcement frequently. You may not be able to have a gun permit. The list goes on and on.
You are thinking right now, but I am not a sex offender, there is no sexual contact. It does not matter. Even the accusation that sexual contact has occurred could ruin your life. Do you have the resources to fight an accusation or criminal charge that you have had sex with this girl, whether consensual or not?
You are an adult. She is a child and will legally remain a child for several years. I can't personally fathom why you think dating this child would even remotely be a good idea.
Remember this is just my opinion and should not be construed as legal advice. No attorney-client privilege will be created by communications on this website or by email. You should consult an attorney in your area about the specifics of your case.