An elder is a wise person who has been through/seen alot. A person who you go to when you need a tough question answered. An elder is a respected and revered person in society. They are helpful and contributing members of society.

An elderly person is old and senile person. They should be in a nursing home or at least watched over most of the day. Elderly people are confused and always talking about "Back when I was a kid" or "In my day". These are the old people steal stuff and dont even notice it. These are the people who drive 20 mph no matter where they are.