How to make toddler and child clothes unisex
 

Just open your package file with SimPE, click in the two places indicated and change the value to 0x3. It should be 0x1 or 0x2 when you start.

Only use this trick on children and toddlers. Older Sims use different skeletons, so their heads might not line up with their bodies and such.

How to "elderize" an adult skin.
 

You can make an adult skin fit YA and Elder too. Just set the Age property (right above Gender in the screen shot above) to 0x58. If you do this on separate tops or bottoms, you need to include a warning that the shirts should only be worn with "elderized" adult pants and vice versa.