From the CIC operational manuals:
In order to meet R2(b)(i) of the definition of “dependent child”, a child must be under 22 years of age and not a spouse or common-law partner on the date when the IMM 1344AE is received by CPC-M, and not a spouse or common-law partner when the visa is issued, and when they arrive in Canada.
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R2(b)(ii) & (iii) describe children over the age of 22 who may be considered dependent children if they are substantially dependent on their parents for financial support. This includes full-time students enrolled in accredited post secondary institutions or children with a physical or mental condition. In such cases, officers must ask for documentary evidence of full time attendance at school, evidence of the institution's accreditation with the relevant authority, evidence of the physical or mental condition and evidence of financial dependency on parents.
Dependent children under 22 years of age
• Is under 22 years of age and not a spouse or common-law partner when the application is received by CPC-M; and
• without taking into account their age, they continue not to be married or not involved in a common-law relationship at visa issuance and when they enter Canada.
Dependent children over 22 years of age and full-time students
Since before the age of 22 or, if married or a common-law partner before the age of 22, since becoming a spouse of common-law partner they have been:
• substantially dependent for financial support on their parents; and
• continuously enrolled and actively pursuing a course of study at an accredited post secondary institution when the application is received by CPC-M and when the visa is issued.
Dependent children over 22 years of age and unable to be financially self-supporting due to a
physical or mental condition. Since before the age of 22 have been:
• substantially dependent for financial support on their parents when the application is received by CPC-M; and
• continue to be substantially dependent upon their parents when the visa is issued.
So as you see from this, as your age is locked in, you do not have to be a full time student to qualify but you shouldn't get married or enter into a common law relationship.
If you had been 22 or older at the time of the application being received, you would have to be a full time student and continue to be a full time student until you would get PR.