Even if your daughter does not have any income to report, either taxable or non-taxable government support, she should file a tax return if she is eligible for Federal or Provincial credits. As an example, a person reaching their 18th birthday will be eligible to receive the Goods and Services Tax Rebate. This is a quarterly payment from the federal government.
It would also be beneficial to the supporting persona (parent) that the dependent adult file a tax return to support a transfer of any qualifying amounts such as Disability Amount or medical expense.
These are but a few examples and depending on your daughters circumstances there can be many more. There is no minimum age to file a tax return in Canada.