In much of the world, especially across Asia, Africa, the Middle East, and Latin America, respect for elders and authority is not a nicety but the organising principle of social life, and visitors are quietly judged on it. Even in more egalitarian Western cultures, courtesy toward age and position matters; it is just expressed less formally.
The common signals are worth internalising. Greet the eldest or most senior person first and let them initiate handshakes or seating. Use titles and family names rather than first names until invited otherwise; in many languages there are also honorific forms of speech reserved for elders and seniors. Stand when an older person enters, offer them your seat on transport, let them begin eating first, and receive anything they hand you with both hands in East Asian cultures or with the right hand in South Asian and Middle Eastern ones. Interrupting, contradicting bluntly, or joking too familiarly with elders lands badly in hierarchical cultures.
Non-verbally, a slight bow of the head, a lowered voice, and not towering over a seated elder all register as courtesy. When in doubt, formality first: nobody is ever offended by too much respect, while too little is remembered.