Hi,
SealJuice and simsbaby requested a tutorial on age conversion in this thread.
At the moment I dont have the time to do a real detailed one for beginners, so I just post a short guide now and try to put in some pictures and more detailed descriptions later.

For demonstration I'm converting an adult mesh to a child one.
Programs used are SimPE v0.58, MilkShape 3D 1.7.9, wes_h's unimesh plugin v4.06 , and demon's extendedManualEdit v1.06c . Big thanks to all the creators :baloons:

The basic problem with age conversion is that every age group (except YA-elder) uses different joints. Just scaling down an adult mesh to child size wont give you a child mesh, cause it still uses the adult skeleton...in fact it would give you a monster :insane:
What we are gonna do is use the joints of a child mesh for an adult mesh

let's start then:
- import a random child gmdc with unimesh importer
- import the gmdc you wanna convert
- two messages will pop up: 'if you import over another model, results may be unreliable. Do you want to continue?' and 'Do you want to exclude additional bone definitions'. press ok both times
note: if the second message didnt pop up, you haven't got the latest version of unimesh plugin. go and get it, otherwise you'd have to delete all the additional imported joints one by one
now comes the actual scaling:
- select the adult body- and morph-group and scale them down by 0.688 (origin)
- move them forward till the adult neck fits over the child one
- select the arms + part of the shoulders and scale them down by 0.92 (center of mass)
- move the arms a bit forward, too till the hands are as close as possible to the child ones
- now align the neck with demon's Extended Manual Edit to avoid gaps (go into top view - set on smooth shaded - check ignore backfaces in select mode - select one pair of vertices around the neck - open Extended Manual Edit - copy and paste the values of the child neck vertices over the adult ones - too keep them apart be sure the index button is checked. in the mesh column every group has a different number)
ok that's it...a bit of fine tune wont hurt.e.g. pull in the breast to make it look more child-like.
Delete the child mesh groups, fix underweighted bones and do the rest with SimPE hope that wasnt too confusing. Any questions, suggestions for better ways... - fire away

Just a tip - to tell which mesh group is which in the Extended Manual Edit tool, look at the number in the first column on the left (if you stretch this column out you will see it says "Mesh")

This number tells you what mesh group it is - look on the Groups tab at the order the groups are listed.

In Extended manual Edit, the first group on the list will be Mesh 0, the second is Mesh 1, and so on.

Thanks a lot! thats very useful. i've always let the index button unchecked...

another tip - especially for conversion from teen/adult to child - consider using some child mesh parts for parts of the body, especially the neck and chest area. That will prevent the wrong proportions and odd breast shapes (I've seen too many child conversions that just don't have child shape to them). Also remember to tone down any under breast shadowing on your textures.