Grandson originally refers to the eldest son of the eldest son, which has nothing to do with age among grandchildren, and later refers to the eldest son.
As for the eldest son and grandson, the earliest record comes from Historical Records Yin Benji written by Sima Qian, a historian in the Western Han Dynasty, which is the earliest and most complete record about the eldest son and grandson found so far. Grandson originally refers to the first boy born to the eldest son. Even a grandson, this boy is a grandson no matter how young he is.
Concept:
Generally speaking, according to the original definition of eldest son and grandson, suppose that Lao Wang next door has two sons, Wang Da and Wang Er. There is no doubt that Wang Da is the eldest son and Wang's grandson is the eldest grandson. Even because of various relationships, Wang Da married late and gave birth late, and Wang Da's son was no older than Wang Er's son.
However, Wang Da's son is also his grandson. Even if Wang Er's child is older than Wang Da's son, it won't be his grandson. In ancient times, we paid attention to the pecking order and were not careless. According to the previous succession order, the eldest son and grandson have unquestionable inheritance rights.
But now with the implementation of gender equality and family planning policy in the past, the requirements for the eldest son are not so strict. Now it is generally believed that the eldest son is the eldest son among brothers, and we are all ordinary people and have nothing to inherit, so we no longer care so much about the eldest son and grandson.