I believe part of the problem is the proportions. If you are going for an Age Regressive effect, part of it is that the head does not change size nearly as fast as the rest of the body. It's not simply a loss of mass and cleavage on the torso either, there's a different curvature as the body ages, which is very difficult to capture unless you know exactly what you are looking for.
This Site
http://www.geocities.jp/denkimorph/anime1/index.html
Has a great deal of examples of how the body changes. There are probably better resources in Lorekeep's archive, but I am sure you can find that very easily on your own.
For programs, I would reccomend PAINT.net, an open source graphic design, which, while not as good as photoshop, doesn't run the (extremely small) risk of having pirated software. I've heard good things about GIMP as well, however I've not tried it myself.
This, of coarse, is all just advice. Take it with a grain of salt.