[tropo]

It's for kids who haven't yet reached age 8 or a height of 4'9" (145 cm). That is the law. They could easily weight as much as their mother.

[brewdad]

Washington state just changed their law. Now kids under 4'9" must use a booster up to the age of 16. Realistically, they have stated that 13 year olds will not be required to use a booster since, at that age, they are allowed to sit in the front seat anyway.