This could be spurred by the fact that your daughter just wants to know her father. If they are asking for back child support, they could force you to take a DNA test. Most likely you are past the statute of limitations, but a family law attorney should verify that. This is not a guardianship issue because the daughter is over 18. If she is your daughter, she is your heir. You may wish to consult with an estate planning attorney because you may want to specifically disinherit her.