The whole world is filled by people whose completion of puberty is delayed, so they stay young forever. Their world is mostly just fun and play, with few people understanding science.
One man's treatment fails and he starts to age. They are horrified by his grey hair and wrinkles. He becomes isolated and mournful. One of his friends or daughters realizes there is more to life, and starts talking to a scientist, and learning the basics of biology.
It's a fairly short story, from the 90s or earlier.