Don't automatically "help" a person you deem as weaker (be it due to old age, disabilities, etc) to do a task if they're already capable of doing it themselves. They already need to deal with the fact that they're not able to many things that they (might've) used to be able to do.
Thus doing things for them will result in reducing their sense of independence and self esteem. This still stands true even if this person is struggling to do something, such as buttoning up their shirt. Just because you can do it much faster and save the other person from struggling, doesn't mean it's the right thing to do.
The feeling of losing your dignity due to lessened abilities can hurt much more than the struggle to do something.