My father was granted custody of me in 2009. There was never a child support agreement between him and my mother. I have never lived with my father nor has he ever supported me. Can my father be sued for back child support now that I am 18 and I live with my mom?
Generally, in order to sue for back child support, you must first establish that there was a valid child support order issued by the court. You mentioned above that a custody order was granted. You will need to check and see if a child support order was filed and granted as well.
If there is a child support order in which your father failed to pay, then you may be able to sue him for back child support retroactive only to the date in which the order was filed. Note, various states have different rules regarding how far back a person can sue for child support owed. Further, since you are 18, your state law may provide you with a narrow window of time to file a legal claim for the back child support your father may owe.