Age casting in K-dramas

Just finished watching Reborn Rich (2022) and the casting according to their real life age felt so wrong. There is the grandfather 54, grandmother 51, their sons 52, 49 and 47. One  grandson 45.............

 Why do they cast actors that are far from biological age what script demands?  Most K-dramas do not have the budget or make-up skills to age people well as was the case in this series.