It's not different in Ontario. The "attaining 16 years of age" you read in the Health Care Consent act is in regard to people being admitted to a care facility and not in regard to people receiving treatment.

For just treatment there is no legal minimum age for consent in Ontario.

https://jcb.utoronto.ca/wp-content/uploads/2021/05/Bioethics-Table-Brief-Vaccinating-Ontarians-Aged-12-17-Legal-and-Ethical-Requirements.pdf

"Treatment’ in the HCCA includes anything done for a therapeutic, preventive, palliative, diagnostic, cosmetic or other health-related purpose, and therefore includes vaccination"

"There is no age of consent for treatment in Ontario. All persons in Ontario who have decision-making capacity have the right to make decisions about vaccination, including 12-17 year-olds."