Depending on what province youre from there are differnt laws. In BC
Children must ride in a rear facing car seat until they are a minimum of 20 pounds (9 kg) and at least 1 year of age. Children who are at least 1 year of age, at least 20 pounds (9 kg) but less than 40 pounds (18 kg) must ride in an appropriate car seat
But in N.S. and PEI you must be at least 22 lbs. Some dont even have booster seat laws! Best to check your provincial governments car seat regulations.