http://www.cic.gc.ca/english/study/study-minors.asp
Custodians
Minor children who are less than 17 years of age and come to Canada to study without a parent or legal guardian must be cared for by a responsible adult in Canada. This person is known as a custodian. In some cases, a custodian may also be needed for minor children between 17 years of age and the age of majority in the province or territory of the educational institution where they intend to study. This will be at the discretion of an immigration officer.
Looks like the immigration officer decided that you don't require a custodian.