Hi
Here are a few of the people who should file a return:
1) if they have any income to report, and applicable deductions to be made, no matter the age
2) a spouse of a tax payer, whose income is $0. This allows CRA to validate the claims of the tax payer
3) students enrolled in post secondary education, to accumulate their tuition credits and transfer some to their parents
4) anyone over 18 years of age, even if just to calculate the HST refund