The six speech levels of Korean language is called "Hearer honorific system(상대높임법)". The speech level is determined according to the person spoken to.
Whereas, adding the ending(inffix), "-시-" to the final ending, is used to show respect to the person spoken about, so it is called "Subject honorifics(주체높임법)".
And replacing the words for honorifics, such as 진지 instead of 식사, is called "Object honorifics(객체높임법)"
If the person who you are speaking to is an elder person or a stranger, you should speak 존댓말, such as 합쇼체, 해요체 and honorifics with "-시-" . if not, you can speak 반말.
손자: 할아버지, 진지(를) 잡수세요. (존댓말)
(The word 진지 is a honorific word for 식사. and 잡수시다 for 먹다.)
할아버지: 오냐, 알았다. (반말)
할아버지: 노인정에 다녀오마.
손자: 할아버지, 안녕히 다녀오세요.
if the person who you are speaking about is the elder, you should use the honorific to show respect for him/her.
When speaking to friends about one's grandfather.
얘들아, 우리 할아버지께서는 연세가 많으시다. (연세 - 나이)
얘들아, 우리 할아버지께서는 연세가 많으셔.
When speaking to teacher about one's grandfather.
선생님, 우리 할아버지께서는 연세가 많으셔요.(or 많으세요)
선생님, 우리 할아버지께서는 연세가 많으십니다.