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=defaultSo 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.