Roughly one in five orthodontic patients is now over twenty-one. Some never had treatment as children. Some had it and stopped part-way. Many finished properly, never wore the retainer, and have watched the teeth drift back over twenty years.
Adult treatment is different in one respect: the jaws have stopped growing, so we work within the skeleton you have. Where the discrepancy is skeletal rather than dental, that is where surgical orthodontics comes in. Everything else moves just as it did at fifteen, only usually a little more slowly.
It is also different in a way that helps. Adults turn up, keep appointments, wear their elastics and clean their teeth properly, which is why adult cases so often finish ahead of the estimate.