You are in luck, although there normally is a 2 year rule for you to disestablish paternity, and you had 60 days to recind your signing of the birth certificate since it was prior to 1-1-1995, that established paternity, upon both parents reaching (CALIFORNIA CODES FAMILY.CODE SECTION 7577) majority, a bill was signed that permits you to challenge paternity established by default for a non biological child, this is limited so you should immediately contact CoCoCo DCSShttp://cc-courts.org/ you can find self help info and other information at this site or the Family Law Facilitator's office ASAP, make sure you prepare a response and meet any deadlines, CoCo County may also allow for telephonic hearings, re your response seeking to disestablish paternity see FAMILY.CODE SECTION 17400-17440. Please understand that if you are proven to be the bio parent of the oldest child you may still be responsible for a large amount of back child support especially if she was on welfare or medi-cal, but since you were not married they should not be able to make you responsible for the other child's child support, but you may still have to fight it. ALso be sure if you are the bio parent of the first child and there is a large arrearage, that they at least discount the portion when you were a minor and possibly more since you were deprived of a relationship with the child or an opportunity to prove paternity by DNA sooner. YOu may retain an attorney to represent you.