When can my child move from a harnessed seat to a booster?

Across the reviewers we read, the consensus is that boosters are appropriate once a child is at least 4 years old, weighs roughly 40 lb, can sit still for the full ride, and can keep the belt positioned correctly. Specialist subreddit threads in r/CPST repeatedly note that most 4- and 5-year-olds are not yet ready, and harnessing longer is generally safer than rushing the transition.