How old should a child be to ride in the car without a carseat or what is the weight the child should be?

This is regulated individually by each state/country, so you should check with the driving manual or on the dmv website of the state/country where you are driving. Typically, there is a weight limit for the child, under which they need to be in a car seat. Above that weight, a seat-belt is fine. 80 lbs is about the norm. Some states have different rules for children under 45 lbs.
Your child needs to be at least 8 or 9.
In the UK, your child must use a car seat until they are 12 years old or 135cm tall (whichever comes first).

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