Good question! If you're an adult with retained baby teeth, they will need special consideration:
1. They're probably going to be more brittle than adult teeth. They may chip or break with braces requiring replacement in the future.
2. They have smaller roots. This means their movement will need to be slow and deliberate. Occasionally if the roots are quite short they may come loose and have to be extracted after treatment.
3. Some considerations will have to be made with regards to their size. Baby teeth tend to be smaller than adult teeth. If they fall in thr front area of the mouth your dentist/orthodontist will want to know what the longterm plan for that tooth might be in order to leave adequate space for a future replacement.
If your question is in regards to a child with mixed adult and baby teeth often times preventive intervention methods are recommended until all the baby teeth are gone.