Please advise me on the CSA

I have a 18 year old son, 19 at the end of May, his mother and I have never used the csa, however she is reaqlly unwell and now is saying she wants to use the csa since he is in full time education and wants me to contribute. I wont bore you with the horror story of her drinking and wild overspending etc, we all have our own issues, however I have always contributed and bailed her out to teh tune of thousands and although I will give my son money direct I legally want to tell her enough is enough, she is threatening using the csa so I wante dto know when does a child become an adult in thier eyes thus meaning they cannot come after me?

thanks I guess someone will knwo the answer fairly quickly, I sigh off a really stressed and upset dad.