My own view on what you are wanting to do is that you will not be prepared emotionally and mentally to cope with any disapointment, either on finding out who your parent(s) might be, or discovering that you may have no known parent. It's more than just a legality that makes being 18 the right age to start the process. You have to have the capacity to understand the possible scenarios, about the past and about a future. You've never lost your birth parent(s) emotionally, only physically, your birth parent(s) have already lost you once. For them it can bring someone into their lives that would mean the world or destroy it.
If you already know something about who you were, then go slowly now towards a time when, if you do it right, you can discover legally that they have left a message for you to find, a message that means they want to be found.
Good luck,