When to make the switch from a 5-point harness to a booster seat?
The switch from a harnessed seat to a booster is often considered a "promotion" or a "Milestone"; however, it is actually a serious downgrade in the safety of children that are too young, small or not mature enough.
The Legal Weight minimum for a booster in Ontario is 40 lbs (the child must be 40 lbs consistently) and you must follow the height, weight and age minimums on the particular booster.
Most booster seats require the child to be consistently 40 lbs, 40" in height and minimum 4 years old. It also requires a proper belt fit which may not happen until the child is larger.
For best practice, in addition to the legal limits the child needs to be old enough to be responsible for their own safety since they need to sit in the correct position at all times, including sleeping (generally age 6+).