One difference is that the treatment takes longer. As you get older, you heal more slowly. As a result, the orthodontist needs to be more gentle with his treatment. The orthodontist can still move your teeth, but the orthodontist needs to move your teeth more slowly to let your mouth heal between tightening appointments. The other difference is that it is harder to enlarge your mouth to get all of your teeth to fit.
Some people elect to have their jaw enlarged surgically.
However, most adults decide to have their teeth straightened, but not get their jaw enlarged. That changes the treatment somewhat because the orthodontist may have to remove some teeth to make everything fit while the orthodontist could have stretched your jaw if you had come in for treatment when you were a teenager. Nevertheless, orthodontic treatment still works. It just goes slower and may be slightly more painful when you are older, because the orthodontist cannot stretch your jaw and teeth need to be pulled. Well, the beginning part of the treatment is harder for adult patients than for teenagers.