When is my child ready for a booster instead of a harness?

Child-passenger-safety techs in the r/CPST community stress that booster readiness is about age, maturity, and fit — not just hitting a weight minimum. Several threads note kids who 'barely squeak by' on height and weight are often still too young to sit properly. Most boosters here start at 40 lbs and 4 years, but reviewers consistently recommend keeping kids harnessed as long as the harness limits allow.