It is very noble of you to want to seek custody of your brother.  I wish that you had given a  little bit more information here like why he is in the custody of the state of Pa while you are all in MD.  If he was taken from your parents for any reason it is doubtful that you would gain custody of him if you were living with the  Now, yes, you would have to be 18 to seek custody.  But it is so much more than that.  You have to show that you have the means to take care not only of yourself but of him.  I would seek some guidance in your area on the matter.  Good luck.