Converting an adult mesh to a teen mesh is not as trivial as it sounds. It involves scaling the mesh properly, adjusting the bone positions, and alot of massaging to get the head/neck seams to line up, and smoothing out the surface normals so the sims don't look polygonal. I spent weeks doing this for the InTeenimater outfits (although Wes_H has done a remarkable job in recent months with his plug-in, so this process may be much easier now - I had to write my own tools for SimPE to do this back then). Granted...if you don't care about certain animations looking poor, hard seams, or small discontinuities in the neckline and such, then you can probably get away with doing the poor-man's solution and scale the mesh down to appropriate size in Milkshape.
Once you've completed the mesh work, you can then export the clothes from body shop. Open the clothing in SimPE, and change the Age field in the Property Set from 0x00000048 (Young Adult/Adult) to 0x00000004 (Teen). You then need to update the 3D References for Resource Node and Shape to point to the Resource Node/Shape in your new mesh file. This is easy to do and takes all of 5 minutes.
I did this recently with a few teen outfits that I wanted to use Warlokk's Teen Body mesh on.
J
PS. Sometimes, if you're lucky, Maxis has "scaled down" teen version of the adult meshes that you can use (or meshes that are very similar) instead of trying to create your own. Don't get me wrong, meshing is actually alot of fun, but as most people who have done it will tell you - it is very time intensive to do it right. It's one of those cases where 80% of the time goes into 20% of the work
