There was something that said in the rules that you still have to LIST the non-accompanying dependents in the application, meaning they still have to be mentioned, even if they are not dependent/accompanying.
Probably this is the reason you got it returned, but the LICO issue would have rejected it anyway.
Should have included a letter at least mentioning why you did not include, CIC are dumb, they have to be told everything.
Don't assume that they will guess why you did not include someone.
Regardless of what reductions they make in age of dependents, it is common knowledge that dependents can easily go over-age when it takes so many damn years to process an application.
Do they want parents to leave their younger kids behind just so they can immigrate ? This is probably why they didn't reduce dependent age to below 18, as in a lot of countries the kids are dependent until they finish university, or reach a certain age like 25, etc. For daughters they remain dependents until they get married, in many countries.