Back child support

I have a quick question… My husband has been paying cs for the last 17yrs, last time order was changed was 11yrs ago. She is getting ready to turn 18 but our question is can the cp take us back to court and get back cs for the last 11yrs? We (us and the cp) have never taken this back to court to modify even though my husband did of course over the years start making more money (as well as the cp was and is making more) then he was making at the time the current order was done. The reason being is we have always just paid the same amount to the cp (per verbal agreements) but yet always give extra money directly to the child, we all thought this was fair. The cp is now saying that she will go back to court and get back cs for those 10 or 11 yrs because he was making more money. Is this possible?

No, she cannot retroactively modify support.