I don't know if you've had or if you have a teenage daughter, but I have. I also spend my days with kids that age. There would be no perceptible difference between the amount she would eat or the amount Marianne would eat. And 13 year old girls are not that much smaller than 16 or 17 year olds either. Many girls reach their full adult height at that age or even earlier. Not much of a savings to be had in terms of cloth purchases, either. She would be no more of a 'child' in terms of how much it would require to support her than Elinor and Marianne are.