I remember reading a book around 1990 that was a first-in-series story in which the World Health Organization released a formula into the global food supply that resulted in every child who hadn't yet reached puberty being made effectively immortal. They still grew up, but never aged. I remember that the protagonist had a brother a year older or so who barely missed having the formula take effect on him. All of the affected children were gathered into communes for their own protection (jealous older people sometimes murdered them otherwise). Eventually the mortal population died off as the immortal generation worked to govern themselves. The implication at the end of the first book was that humanity was in trouble because immortality made people less interested in progress and enlightenment. Also, there was a child with strong sociopathic tendencies who was being set up as a future villain in the series. I think the author was a woman, but I can't recall her name, and I have no idea what the title was.
It's the Earth Song trilogy by Sharon Webb. The individual books are:
- Earth Child (1982)
- Earth Song (1983)
- Ram Song (1984)
You've got the details very well. When they realize that creativity is blunted by becoming immortality, a project is started where the talents of gifted kids are nurtured. The kids are then given the choice--keep practicing your 'art' or give it up and become immortal.