I have paid non court ordered child support since my son was 3y.o. He is turning 18 this year and I have a feeling that my ex will now ask for a back payment for the first three years. Do I have to pay this? Also can she take me to court to pay this even thought she has never ask for back support in 15 years?

Do you have proof that you have paid child support for all those years and that the amount was meeting state guidelines ?

CA generally does not allow support retroactive before a petition for support was filed.