FRIENDS COME IN BOXES

Sphere paperback, 1976. Cover artist uncredited.

"The problem of immortality was solved in the 21st century: at forty, your brain was transferred to the head of a six month old child. Thus you gained another forty years of active life, until you could do it all over again. 

But then the birthrate fell - and a growing horde of brains waited in the friendship boxes for host bodies. And on the sidelines a rebellious minority fought the system, seeking to raise their children in freedom to live a normal life. 

Inevitably, the factions moved towards confrontation: a day of final reckoning."