In order to bring more in one class, it would have to cut out more...?
The problem with having an unreasonably low age of dependency, such as 18, is that Canada is losing out on some families. In my own experience in my practice, I have seen this. Once they hear a family—say they are skilled workers and they may have been working here for a while and their children are going to university in Canada, for instance, and making their home in Canada.... When I tell people that when they apply for permanent residence they will have to choose whether to stay in Canada without their older children or go back home, I have had clients who have decided that they can't split up the family and will simply choose some other country where they can remain together as a family.
I don't have any hard statistics on this, of course, but I think what Canada is losing by not allowing entire families to immigrate should be taken into account in the study you are doing .