For me, 40 lbs. is a minimum in terms of weight. As for age....that's more a matter of the child. And, your circumstances factor in as well. We don't carpool for school and it's a VERY rare occasion when someone other than dh or I drive our kids, so I wasn't as worried about them being able to ride in a booster for short trips in those instances.
My older son was six before I felt like he was ready to start booster training, and even then, it took over a year before I felt that he was mature enough to booster full time.
My daughter has done MUCH better in the booster. She just turned five and has needed relatively few reminders to sit properly. I had anticipated needing to go back and forth between the booster and the harness for longer trips, but last Saturday I kept her in the booster for a longer trip and she did wonderful (even fell asleep and maintained position perfectly in the Monterey...it's a great fit for her). We'll be taking a fairly long drive in a couple of weeks and I'll probably harness her for that for my own peace of mind.
Her twin brother is still harnessed....he's got a few pounds to go before he hits 40, and he's less mature as well. I don't think he'll be 7 like older brother was, but he needs a little more time to mature.