The age of majority in BC is 19 for civil law.
So, if an agency hired an 18 year old, or if someone paid an 18 year old for sex, in theory the parents of that person would have a legal right to sue the agency or the SP's customers.
Knowing the BC court system, some large amount of money would be awarded.
Why would an agency take the risk?
Also, while I haven't read the latest version of the age of consent legislation (when they recently raised it back up from 15 to 16) I do know that in the previous version, sex for hire and sodomy had an age of consent of 21. Not sure if those provisions are still in there or not, they may have been taken out since I last read the legislation which was over 10 years ago. There was also a provision for sex where one person clearly was in a position of power over the other, having an age of consent of 21 as well (for example, a 20 year old university student having sex with her prof was illegal previously).