For emergency use-when they meet the requirements of the booster they are riding in and can sit correctly short term. For us, we needed a booster for when I went into labor. I didn't have a seat compatible with my parents car, nor was I going to manuever around in their car while 9 months pregnant and in labor (I did it once, wasn't gonna do it again!). E was 4y7m and 33 lbs and did perfect in the booster the three times we let her ride in it prior to me going into labor(we wanted her to have a few rides so her first time wasn't when she was with my parents. Never once moved and she fit perfectly. However my mom never went anywhere with them when I was in the hospital so it was a moot point. My parents also can't do a harness if their lives depended on it. They buckled the booster when I asked them to show me and did just fine.
She won't ride in it again except in some extreme case until she turns 5. She'll still be less than 40 lbs but if I wait till she's 40 lbs she'll never be in a booster
So for me-emergency use-4 years and meets minimum requirements for the booster
For part time and booster training-5 years and meets requirements
For full time use for normal children-5.5-6 and meets requirements
I don't put a weight limit on it. All children are built differently. To me, being 40 lbs doesn't make her magically safe. Fitting properly in the booster does. There are plenty of 40 pounders who don't fit properly in their booster. And plenty of 36 or 37 pounders who do. So which is safer?
I do age and maturity and proper fit. My child won't be 40 lbs when she goes full time into a booster but she will meet all the requirements I have. She will be old enough in my book(5.5 or so), mature enough, and will fit properly.
Others may not agree, but that's my stance.