You can use baby wipes on the elderly when changing adult diapers, wiping their face etc.
While baby wipes can be used on the elderly they do make larger wipes for adults that are easier to use on the elder and especially when changing an elderly persons adult diaper.
Baby wipes can be used on anyone even when you're not elderly.
I use baby wipes on myself to get cleaner after pooping in the toilet and they work better than toilet paper.
I use the toilet paper to get most of the poop off and then I use the baby wipes after to clean up again to feel fresher.
If you use baby wipes to clean the elderly persons bottom during a diaper change and they pooped then it will take more baby wipes than adult wipes.
I would personally use adult wipes for the elderly as they are larger and work better but baby wipes or even a washcloth will work.
The adult wipes are usually slightly more in cost than the baby wipes but you will use less so it works out pretty evenly.