At the very minimum, for a belt positioning booster 4 years and 40 lbs. BUT a child is much safer harnessed for longer. There are some boosters for 30 lbs and up but I still have my 5 year old who is around 35 lbs in a harnessed seat and will keep him in one until he is over 40 lbs.
When they've outgrown every 5pt. harness available
My dd will be in a rear facing seat to the upper limits (33lbs) She will be in a full carseat (boulevard) to the upper limits (65lbs or height) then she will be in a harnessed booster to its limits (at this point planning on the Regent- goes to 80 lbs) so there's no way she'll still be a toddler when she's out of that
Absolutlely, Angela is right again. They should remain in 5 point harnesses as long as possible. My girls ride in Britax Regents. The Regent is a fabulous seat, well made, safe, and, my daughters tell me it is very comfortable. It has that great Britax quality.
Here are a couple of good deals on Britax Regents: http://tinyurl.com/gre3l
This is a steal -- a beautiful new Regent in Blue Bouquet for only 199 including free shipping. I paid 249 each for two of the same exact seat just a few short weeks ago. Sadly, this is the girly print, the GN prints are still 249 there.
Wow, make that Baby Universe Regent Deal even better by going through ebates.com: http://tinyurl.com/maozb and get six percent back (I guess that would be twelve bucks) plus another five bucks for signing up: so that would be 17 additional dollars back, bringing your total to 182 for a Regent in a girly print including free shipping.
I just paid 498 for two of the same seat, and that hurts