I read this novel in about 2020. It seemed to have been written pretty recently.
Any child up to the age of 18 could legally be given up with the permission of their parents and be killed.
I am pretty sure their organs were donated to the sick. I am not completely certain about this detail.
The protagonist is a teenage boy sent to be killed, this is arranged by his mother and stepfather. Naturally he feels horrified and betrayed. He runs away.
He meets other young people who are meant to be killed in this way.
One such boy sticks in my mind. His family are very religious and consider donating his life a tithe. The boy himself is convinced that dying is the right thing for him to do. His family throws him an amazing party before he goes off to die. The Pastor of his church tries to save the boy's life by telling him, in confidence, to run away and hide until he is over eighteen. His large family have adopted and are raising two kids who were left on their doorstep as newborns. One of their own flesh and blood they send to be killed.
There is some reason why leaving newborn babies on doorsteps is very common in this society but I do not recall it.