We are seeing this problem increase as new creators come into the fold, so for those who wish to create clothing for Sims3...Categorizing clothing as Teen through Elder should not be done. Teens have a different body shape from the Young Adults/Adults who have a different body shape from Elders. Teen clothing should be on Teen meshes, Young Adult and Adults share the same body shape and can be on the same mesh, and Elders should be on Elder meshes. When trying to categorize clothing in multiple age groups, there will be problems with it fitting correctly. With tops, the neck and waist will not match up. With bottoms, the waist and feet will not match up. With full outfits, the neck and feet will not match up. Look at the attachments below to see what can happen when age categories are selected that shouldn't be. The examples below are Young Adult/Adult meshes shown on Teens and Elders.
So make sure to stay in the correct age category for the mesh you are working with.