National Hit - Nation - includes everyone, all ages.
Youngsters actively seek out new experiences, whereas as you grow older you get more selective about what you consume- hence it is more a deal to impress older people.
A true hit would be able to achieve that across age groups.
Statistically it is well established that Kpop's consumption drastically drops once people enter college, jobs etc.
So all the kpoper should remember that time will not spare them either.
However even the genre will age, because people who loved the songs of their 20s will keep them as the benchmark-nostalgia even as they age, so the 1st gen 2nd K-poppers, who would be in their 30s/40s would still enjoy them, but may not enjoy 3rd gen/4th gen - nostalgia value is huge.
That is also why all consumable industries actually prefer to target the teens, because that is the age where most our tastes/worldviews are firmed, sure there will be some evolution after that, but the base is the food liked, the music enjoyed etc. in the teens, for the majority.
So a truly national hit will be popular with both the teens and the older groups.