it is clearly mentionned in the guide, Did you read it ? http://www.cic.gc.ca/english/information/applications/guides/3999Etoc.asp
Definitions of dependent child (types A, B and C)
Your child or the child of your spouse or common-law partner can be considered a dependent child if that child meets the requirements of types A, B or C below:
The child is under the age of 22 and is single (not married and not in a common-law relationship).
The child has been continuously enrolled in and in attendance as a full‑time student at a post‑secondary institution accredited by the relevant government authority and has depended substantially on the financial support of a parent either:
since before the age of 22, or
since marrying or entering into a common-law relationship (if it happened before the age of 22).
The child 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 himself or herself because of a medical condition.