Age 51 - Wanting Invisalign but Still Have a Baby Tooth

A baby tooth like yours can develop a toothache and infection at any age without undergoing any orthodontic treatment.  On the other hand , if it is not moved much in treatment it will probably be OK.  With braces you can literally omit placing a brace on the tooth and it will then receive no force at all.  With Invisalign you can make the computer simulation appear to not move any particular tooth.  In reality, all teeth in an Invisalign tray serve to anchor all the other tooth, so there may be slight movement even though the computer simulation makes it appear that there is no movement on a particular tooth.  The bottom line is that I think you should be fine getting Invisalign and don't worry about your baby tooth.

What you describe sounds like the result of a missing permanent lower 2nd bicuspid tooth.  This is a very common tooth to be missing and the baby tooth it is supposed to replace can sometimes last decades and even for the rest of a persons life.  If it is lost at some point in the future, an implant would be a great option.  The Invisalign could work and be gentle on the baby tooth as all aligner movements are very slow and incremental.  The other problem you suggest is that of a narrow upper jaw.  Today surgery is not necessary for the correction, but a removable appliance with Slow Palatal Expansion can do the job.  Not all orthodontists are familiar with this technique or even agree with it, but some of us have successfully treated thousands of adults with this non surgical approach.

It is important to determine why the baby tooth is present.  If the adult tooth that should be there is missing, and there is not excess crowding, it is usually best to keep it.  If the adult tooth is present in the jaw under the baby tooth, you may want to extract the baby tooth and bring the adult tooth into the mouth.  As to the possible narrow jaw, if the bite is not causing problems with your teeth or TMJ, I would not have surgery but move the teeth into as straight a position as possible to improve appearance.  

The baby tooth will not cause problems with removable clear appliances such as Invisalign, or fixed clear braces such as SimpliClear as long as care is taken not to put too much force on the baby tooth.