It has nothing to do with height and weight.This is why so many kids are riding in unsafe seats.
I keep my kids in a 5pt harness until they are 7 and they are tall for their age.
If your child ever falls asleep in the car, reaches for stuff, wiggles around in their seat or turns around in their car seat. They are not safe in a booster seat. I don't know any 5 year old that sits forward and still enough to be safe in one.
I had my 5 yr old in a booster. Its not the age, its the height/weight.
I wouldnt put a 5 yr old in a booster let alone a 2 year old. A 2 year old should still be rear facing.