没有这个说法，你可能听错了。12岁以上的孩子可以自行申请，12岁以下的孩子由监护人申请。只要申请就可以办理，没有不用办的说法。
Minors applying for themselves
Children who are 12 years of age or older may apply for their own SIN.
If you are under the age of majority in your province or territory and applying for a SIN, you must provide all required documents. If you cannot provide all required documents, a representative must apply on your behalf.
https://www.canada.ca/en/employment-social-development/services/sin/apply.html