Best practice is to leave a child rear facing until the limits of the seat they are in, or until at least age two. Bare minimum by law is to consider provincial law (1 year AND 20lbs in BC) plus however a particular seat is labeled (i.e. you must “properly use” a restraint to legally use it in any province; any new seat for sale today requires a minimum weight limit of 22lbs, most require a 1 year minimum, and some require a child to be walking unassisted). Aim to keep the child harnessed until the limits of the seat, and at least 40lbs and developmentally ready to move to a booster. The legal bare minimum in Canada is 40 pounds.
When installing, aim for 1″ or less of movement at the belt path when installed with either UAS (latch) or seat belt (but never both at the same time).
By law, the seat must be top tethered tightly to an approved anchor point in the vehicle. If your older vehicle does not have tether anchors installed, you may be able to have them retrofitted. You must use an approved part from your vehicle manufacturer, never an after-market part from a hardware store. Contact a tech for more information or for the part number for your particular vehicle.
When harnessing your child in the seat, make sure that the harness is AT or ABOVE shoulder level. The harness should be tight, so that you cannot pinch a horizontal fold of harness at the collarbone.
Forward-facing seats are outgrown when the child approaches the height or weight limit of the seat, OR the tops of the ears are level with the top of the hard shell, OR when the top position of the harness is no longer at or above the shoulder level (whichever comes first applies).