In Korea, who the superior is and who the inferior in any relationship is highly important even for the basic mechanics of how sentences are constructed. It would take a while to fully explain this. I can say the same exact same sentence in lots of different ways, altered depending on my relationship with the listener(s). It means constantly having to evaluate relative positions within a hierarchy, shifting forms as context dictates. I told you it's complicated.
One layer to this (certainly not the only one) is titles. Koreans will generally always use titles for anyone higher in a hierarchy; many times people don't even know each other's names because they just cruise along using titles.
Age is one of the most powerful natural hierarchies in the Korean mind. Here's how it works:
Don't think this is an anachronism. Even for those born in the 2000s (that is, school-age children in 2014) the system shows no signs at all of cracks. The children observe it loyally and naturally.
There are lots of problems in logical consistency that occur to me with this system. What if one boy is born January 1st and one December 31st the "previous year"? East Asians traditionally determine age by year of birth (everyone born in the same year is the same age), so even separation in age by one day calls for use a special "older brother" title. Not using the title for an elder sibling would be a serious faux pas; very rude. (Koreans whom I have quizzed about this -- "Have you ever called your [elder sibling] by his/her name?" -- They generally promptly say "never". Upon further reflection some will say, "Maybe once or twice when I was very angry".)
Back to the twins problem. Two twins, one born at 7 PM and one at 8 PM. According to Korean thought, our textbook says, the brother born at 7 PM is the "elder" but that "many European countries" believe that the twin who was born later is the elder. I don't know of any Europeans who would obsess over this matter. Nor had I ever thought about this problem (of determining elder status between twins) before. I am tempted to insert a "so-called" before the word "problem" in the previous sentence, but far be it from me to be culturally insensitive!
The lesson is that Koreans show a shocking level of commitment to the principle of hierarchy. "Come hell or high water".