Asking a tax preparer is never the wrong answer, and if you're unsure you should maybe consult them, or call the CRA help line at 1-800-959-8281.
I used to work at CRA years ago, and the information you're looking for is readily available online. You just need to know where to look (it's sometimes hard to find).
To receive the GSTC, unless he has a spouse or common-law partner or a child with whom he lives, he will need to be 19 years old on the first day of the month the payment is issued (see
https://www.canada.ca/en/revenue-agency/services/child-family-benefits/gsthstc-eligibility.html).
So if he turned 18 in April 2021, he will turn 19 in April 2022, which makes him eligible starting the July 2022 payment. He will need to file a 2021 tax return next year in order to receive the payment.
If he turned 18 in March 2021 or earlier, he will be eligible starting the April 2022 payment. He will need to file a 2020 tax return to receive the payment. (see
https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/rc4210/gst-hst-credit.html#P130_10387)
But keep in mind that if he has any income at all, it might be a good idea to file taxes anyway, especially if he had any deductions taken (taxes, CPP or EI) as they would likely be refunded to him. Additionally, he could start clearing RRSP & pension room for future contributions. I've been filing taxes since I was 14 years old specifically for the the RRSP reason on the advice of my mother, who also worked at CRA.
A complete list of *who* should file taxes is found at
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/you-have-file-a-return.html
Hopefully that's helpful.
edit to correct a factual error and add link to correct info
Edited by Abbysmum, 2021-04-16 11:43:30