The word "seishun" ("youth") comprises the kanji characters for the words "blue" and "spring," which connotes the freshness and innocence of those earlier years in life. It's also the name of a genre of anime and manga. Even as the population ages, seishun manga and anime seem to lock themselves to the Japanese psyche — where as long as your favorite seishun series exist, so does the illusion that youth is always within reach, no matter how old you are.
It should be noted, though, that even if seishun characters never grow up, this doesn't mean their manga and anime series continue forever. The unspoken agreement among most creators and their audiences is that seishun stories will end once the central characters turn 18 or graduate from high school. This formula is especially applicable when sports come into play.
Seishun drama and sports is an unbeatable pairing — every character is gorgeous to look at and brilliant at whatever bukatsu (exra-curricular activity) they choose to do. The blood, sweat and tears of it all is enhanced further by the energy that comes with being agile teenagers, able to do amazing things on the field, in the gym, on the court and in the pool. Once these characters hit 18, however, the glory days almost always come to a close.