The biggest factor in this question is usually insurance. Check with your insurance agent to find out if there is an age limit they require you to abide by.
Another big factor is, of course, potty-training. You may have a child who is legally old enough according to insurance policy to ride the bus, but if they are not potty trained I don't pick them up. This sounds harsh, but if when you start changing children's diapers that also opens you up to legal danger. Not only that but when you have to worry about a little kid wetting their pants every week, that diverts your attention away from the other kids as well and turns out to be more of a hinderance than a blessing to you and the child.
You may have already had your question answered before this but if not I hope to have helped you somewhat.