The correct answer is customs
If we first accept a general definition of custom, by saying that it is a traditional and widely accepted way of behaving or doing something that is specific to a particular society, place, or time, then reverencing (presented in Japaneese culture by bowing) it’s very traditional to Japans Culture. It is say that bowing started sometime during the Asuka and Nara periods (538-794 AD) after Chinese Buddhism was introduced, as a way to show status. In the present, it is used in a variety of occasions and reasons, including demonstrating respect.
This being said, it became a custom to reverence elderly people, even to the point of having a National Day known as keirō no hi.
