I read these in the mid to late 80s, to give a frame of reference. The basic plot, as I remember it, was that a group of scientists released something in to the water supply that turned all children below a certain age immortal. The initial story dealt with the implications of this and the reaction from adults to it. The story followed the main character, a young boy, and his older brother who did not become immortal. In a later book, second or third maybe, it was discovered that the immortals lost their creativity and so it was eventually discovered how to reverse immortality so that a group of mortals could be born who were able to create music and poetry etc. I remember there was an antagonist who created a cult following as well. My Google searching is coming up blank, but it did lead me to here. Does any of this seem familiar?