Quoting Andie.:" The bare minimum recommendation is 4 years and 40lbs, but maturity is a huge factor. If they can't sit properly 100% of the time, they are not ready for a booster. If they are not sitting correctly, the belt could injure them in a crash. Most kids just plain do not have the impulse control until closer to 5 or 6."
Here, it's Law to be 4 Years AND 40lbs before Boostering.
At 5 My Son keeps bouncing around between 38 & 39lbs. Which is fine, he likes to sleep, & he's far safer Harnessed than Boostered. Actually he wants to Rear Face again Like His 2 year old Sister, she's so Relaxed & Comfy in her seat, & doesn't have to have her Legs hang down or be unable to Cross 'em. When they sleep their Head doesn't always stay up ('though he is finally starting to have it stay up now), so they're definitely not ready for a Booster. I've even put my Little Cousin(7years) back in a Harness when he's been with Me, because in-spite of the fact he's been Forced into a Booster for the last 3 years, he really isn't ready for it, nor is his 8.5year old Brother(Legal No Restraint age is 8years AND 80lbs, but his Parent's only followed the 8year rule). Now with our Radian's there's a Chance he'll be Boosterd or Harnessed again too, with us anyway
I Love My "Other" Boys, & Want to treat them the same as My Own, and so I DO when they are With Me!