There is no minimum age limit on who should file a tax return. The link below gives details on who is required to file a tax return, but basically, if you earned income you are expected to file a return. Even if your niece didn’t earn income, if she has eligible tuition, she would want to file a return to be able to transfer or carry forward that amount.
https://turbotax.intuit.ca/tips/who-is-required-to-file-an-income-tax-return-126
She did work during the year but when I enter her t4 info in it shows she owes taxes. I was under the impression that if she was under 18 she would be refunded any income tax that she paid. Can you clarify that for me? Thank you