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I'm really interested in MH's character setup itself. I'm wondering: how is a character aged up and down? I understand there are a lot of targets to move through. But in the program itself when you adjust the slider, what is happening? Is it just applying a target morph? If so, are the bones for the skeletons applied after that in some way? Or is the character scaled with the joints and then the morph applied? Really curious. Trying to recreate this effect in Blender or Maya.

Age is a macro modifier, i.e a set of targets. Different morphs are used on different parts of the scale and in different combinations with for example muscle and ethnicity. See this folder for an impression of the set of target's we're talking about:

https://bitbucket.org/MakeHuman/makehum ... at=default

So if you scale between ages 1-10 (for example), you'll be morphing between "baby" and "child" targets, whereas if you scale between 10 and 20, you morph between "child" and "young".

The skeleton is adjusted in correspondence with the target: the macro targets contain "joint cubes" which are normally not visible (but you can load the base mesh obj in a 3d modeling studio if you want to see them). The skeleton is adjusted so that bone lengths match the positions of the joint cubes.

Thanks so much for the quick and detailed reply, Joel.

Also I took a look at the base obj file. When the character gets scaled (literally?) with the age modifiers, are the joints constrained to some surrounding vertices to maintain their position? Or are the joints the things that are scaled up/down and then the modifier is dialed in/out afterwards? Or are the joints and mesh/targets scaled independently?

I'm pretty certain the joints are vertices like any other vertices in the base mesh. They are affected by the same targets as the rest of the mesh.

The character isn't scaled in the sense that the mesh object as such is scaled. Rather, all targets implement a shift in specified vertices in individual directions. In the case of age, that direction happens to in many cases be upwards. The joint cube vertices are shifted in the same way, in the same process.

("pretty certain" above because I'm not the one who implemented these parts, and might have misunderstood things)