You will likely be considered the psychological parent of the child now after 6 years. You can ask for biological paternity testing. However, a judge may deny it. Courts are extremely concerned about the fact that kids know who their parents are and do not get confused by a change in their parent. Talk with an attorney for a consultation for advice.
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I agree with Mr. Leroi. You are likely to be considered the psychological parent even if you are not the biological father, after 6 years. If a court finds you are the psychological parent, then you may still be required to pay child support. Also, it depends if you have admitted paternity in any of the court proceedings so far. I would think long and hard about going this route. However, if you are certain that they are not your children, you could petition the court to order a paternity test. Whether they would at this juncture is hard to know without more details of your case.
Legal disclaimer: Answering this question does not establish an attorney client relationship. This answer is for educational purposes only. You should consult an attorney in your state for your circumstances.
If paternity has already been determined in a judicial proceeding, it may be too late. You would be well advised to speak with a family law attorney who is well versed in paternity matters to determine whether you have options at this point.