Looks smaller.

Kids have proportionally bigger heads and hands/feet.
Also, the face is smaller compared to an adult's (making the front bigger).

Of course it depends on what you mean by "young" and "old".

If she's supposed to be 18 and 40, her overall body shape would be pretty similar, but with broader hips and heavier breasts (depends also on children - did she have some? How many?). There is also an age people start growing larger, if we're speaking about modern middle class people.
Your robe suggests something older though, in which case her wealth and era would be important too. Women changed with the canons of beauty of their time: Everything from lithe and "youthfull" to fat and flabby was considered beautiful at some time, and grown-up women tried to adapt...