Need some expert opinions here. Does this look like a yaonger and older version of the same person or just a larger and smaller one? I'm wanting to do a series of pictures of the same person over a period of time so she needs to age. If you think she just looks smaller, what do you suggest I do to fix it?

loose the breasts and the pointy chin, make the brows a bit heavier

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...

I was interested in this too and googled "human proportions" to get a better idea of what I should shoot for. I can't remember the numbers exactly. Plus the eyes of an infant and of an adult are almost the same time, even thought body length is different. Some head / body ratios:

Infant 1:4
Toddler 1:5
Teen 1:7
Adult 1:8

These are not absolutes and some people would say that adults are 1:9 but I don't see that in day to day life.  Also younger people have a higher percentage of body fat which rounds out the facial features.

I usually follow this (by the ever awesome Loomis)

the kids arms and legs need to be relatively shorter than the trunk. Love esther

I should clarify, the ages are about 17 and 23 (with no pergnancies). Modern middle class aging is what i'm going for. The heights are 7.5 heads for the 17 year old and 8 for the 23 year old. Same as what tekmonk said.

Maybe a point to keep in mind is that a girl of 17 and a woman of 23 don't necessarily have the height difference that males have at the same ages. Differences are more subtle - a girl of 17 might have less defined facial features, less difference in the breast/waist/hip ratios, but put her in a formal dress next to a 23 year old and many wouldn't be able to tell which is older or by how much. My daughter reached her "adult" height at 14 years old (she's now 22), my youngest son (now 17) is still growing.

Looks like a miniature version to me, not younger & older. Heads, hands & feet should be about the same on both. Also, the younger one should have her eyes lower on her face than the adult (as we age, our eyes raise). Adults also have larger noses and ears (cartilege). hth

Head size is not going to change with age.  Hips need to be much narrower.  Shoulders need to be somewhat narrower.  Hands need to be bigger.  Thinner arms.