Definition of dependent children
A child must meet the requirements of type A, B or C below to be considered a dependent child:
Type A
He or she is under the age of 22 and single, that is, not married and not in a common-law relationship.
Type B
He or she married or entered into a common-law relationship before the age of 22 and, since becoming a spouse or a common-law partner, has
been continuously enrolled and in attendance as a full-time student in a post-secondary institution accredited by the relevant government authority; and
depended substantially on the financial support of a parent.
or
He or she is 22 years of age or older and, since before the age of 22, has
been continuously enrolled and in attendance as a full-time student in a post-secondary institution accredited by the relevant government authority; and
depended substantially on the financial support of a parent.
Type C
He or she is 22 years of age or older, has depended substantially on the financial support of a parent since before the age of 22 and is unable to provide for him/herself due to a medical condition.
http://www.cic.gc.ca/english/applications/guides/5289E11.html#wp1051995