"Four and forty [pounds]" is the best practice, assuming a child is otherwise mature enough to sit properly in a booster. I would get a harnessed combination seat, though (harness that converts to a booster) given that you have a 1.5-year-old to hand it down to. Check the expiration dates on your convertibles--they likely won't last you till your little one is ready for a booster, so you'll have to buy another harnessed seat at some point anyway.