Age in itself is not a limitation for orthodontic tooth movement. Rather it is the health of the periodontium that is more important.
The age limit for younger patients has to do with when the last permanent teeth erupt (usually in the early teens) with the exception of the third molars. Typically this would be around age 13 as the 2nd permanent molars are erupting at age 12.
Treatment at this age should be approached carefully as eruptive movements will cause aligner fit issues.
Some older patients need to be reminded that metabolism, healing and tooth movement are slowed relative to a younger person.
In addition, the rate of tooth movement is highly variable and one cannot "guarantee" that treatment will proceed at a specific rate.
Older patients generally have more dental work relative to a younger patient. The dental work includes crowns, root canals, veneers and other restorations. For the most part, these items are not a contraindication for tooth movement but the patient, as part of the informed consent, needs to be advised that during orthodontic treatment, the restorations may fail or the root canal treatment may flare up.
Fortunately these situations are relatively rare but one must refrain from determining a direct cause and effect of this failure with orthodontic treatment because the reasons for failure are many. The restoration or root canal therapy could already be compromised and orthodontic movement is the straw that broke the camel's back. Further, the failure could be coincidental with the time of orthodontic treatment.
For example, endodontically treated teeth become more brittle with time and are prone to fracture. Another example is a crown that comes off during treatment because the cement is failing.
Additionally, orthodontic treatment changes the occlusion and the new occlusal scheme may produce interference and heavier contacts that may exasperate an already compromised situation.
Nevertheless, when a restoration or root canal fails during orthodontic treatment, patients almost always blame orthodontics as the culprit.
For this reason, it is important to inform patients in advance that these risks are part of orthodontic treatment and that some crowns and restorations may need to be redone to restore proper occlusion since these were designed for the previous occlusion.
Lastly, older patients have more wear and tear on their dentition. Often there is uneven incisal edge wear, worn cusp tips, tooth chipping, craze lines and tooth discoloration. It would be prudent to identify these issues in advance and let the patient know that restorative dentistry has a number of solutions for these issues and these treatments can be coordinated with orthodontics as a comprehensive treatment plan to restore and rejuvenate their smile.
If you are treating an older patient with clear aligners please be sure to make them aware of the above information prior to starting treatment.