From your post, it sounds like you want to do a private adoption without the birthparents involvement. Unfortunately, it's not possible to go around the birthparents' rights. They have to be notified of your intention to adopt & you have to have their consent. And like I said, if you want to go the "I can't find them" route, it has to be an honest "I hired a private investigator and here is everything we've done" kind of situation. You have to convince a judge (who doesn't terminate parental rights on a whim) that you did everything humanly possible to contact that birthmom
To help you on your way there are a few people you will be in contact with. Here we will describe the roles some of these people play. To adopt a waiting child or teenager, you will work primarily with an adoption agency. It is only at the end of the process that you will need an adoption attorney / lawyer, who will prepare the paperwork to be filed and represent you in court.
I think in PA it is 18 but I will check into it. I'm very open with her and she could see her birth mother now if she wanted to and we could actually locate her. She met her three siblings last year and now keeps in touch with them. My adopted son (now 20 years old) has no interest in meeting with his bio mom or siblings but I've always been open with him and left him know it was an option.
One point that Matt mentioned that I found interesting, and certainly gives the service credibility, is that they have lawyers that use their service as well. The lawyer will sell the service to a client for thousands of dollars and then pay RapidAdoption.com to prepare the paperwork for them. There are some that they work with regularly and have an agreement with and some that just purchase the service from their website and give the paperwork to their clients as if they had done it.