Depends on where you live.. In the Georgia, Tennessee, Alabama area you have to be 4'9 or 9 years old
If I remember right, it's not a matter of how old, it's a matter of weight. I think it's something like 95 lbs? I'm not certain. Use good judgment. If the child (or whatever you're putting into the seat) is mature enough to not sit in the seat, don't force it.