We must respect the elderly as how we want to be respected when our time comes.
1. The teachings of Confucious would say: "Do not do unto to others what you don't want others to do unto you."
2. The teachings of Christianism from the Holy scriptures would say: "Do to others what you want others to do unto you."
3. The law of Inertia in Science would say: "In every action there is an equal reaction." (If the adult would show disrespect to the elderly his children might do it to him when he grows old. As it becomes a value system).
4. The Hinduism would believe that good things bears good karma.
Regardless of having religion to non, it would still be fall on your character, the kind of person you are.