My rule of thumb with kids is that once they regularly zip past you and engage in toe-tapping waits, it is time to give them a bit more to carry. By the time they are about 12 years old, they can carry their full share. But keep in mind that their growing bones cannot safely carry huge percentages of their body weight - nor should they ever have to. It is good to teach kids what they NEED vs. what they want. An early teen should be able to carry thier basic NEEDS. PS- electronic gagetry for their entertainment is NOT a need!

Another rule of thumb is let them walk their age- 2 miles for a 2-year old (aware that they have no compulsion to walk steadily in a straight line and are more interested in looking at plants and rocks along the way). You end up carrying them part the way, not because they cannot do it, but you simply need to get there in a reasonalbe time. Children walk well when they do this regularly at home. Bribes work well - it always took a dozen cookies to entice my 2-year-old to walk the 2 miles.