You would open a new account, or change your user name, if you want to be answering things from a more professional viewpoint and no longer have your answers associated with what you said when you were on the site at age 15. I believe that if you move to a new user name, someone who is interested might be able to look up your profile and find the old one (it used to be that way on MedHelp but I don't know if it works like that any more), but only a really motivated poster would do this, most people just want someone comment on their question. And in a lot of ways it does not matter anyway if they read things you wrote years ago, everyone grows and changes, especially someone who started posting on the site at 15. who is now pre-med.
Actually when you do became a medical professional we have a screening process that we do. Simply contact us and let us know when you are ready :) Thanks for being a part of our community for so long, we certainly value your loyalty.