When did you drop the 5 point harness?

Minimum for the belt I believe is 4 years and 40lbs and when you can pass the 5 step challenge for belt position:
https://thecarseatlady.wordpress.com/boosters/5steptest/

Recommendation is to keep them in the 5 point harness as long as the harness fits. We just bought the Britax click tight frontier and that could be a while because its one of the highest rated combo seats.