That age changes depending on where you live. As a minor, you would be under the care and control of your adoptive parents. If they don't want you to see a blood relative, you are supposed to obey their wishes. The age of majority is set by the country, province, state, or area in which you live.
Check out the age limit in your area. Being of majority age will help if you need to contact any agencies to help you in your search. It is doubtful, if you wrote to any agencies to find the address of relative or any other information you might need, that they would respond.
If they are allowed to release the information, it is usually only released to an "adult" person who can show a need to know.