Our society tends to view children as more fragile than adults, and there is good reason to do so. There has been a tremendous amount of research over the years demonstrating that the human brain develops slowly and children'd brains work very differently than adult brains. Even kids aged 18 have less of a capacity to think well than adults do.
The Court stated that it is a violation of the Eighth Amendment (Cruel & Unusual Punishment Clause) to sentence a juvenile to mandatory life without parole. A mandatory sentence implies that a court has no choice but to sentence every juvenile convicted of certain crimes to that harsh punishment, regardless of the child's individual circumstances.
It is still possible to sentence a child to life without parole, but those individual circumstances must be weighed--the age of the child, the developmental level, the quality of the parenting--all of these are important aspects of child development that have a much stronger effect on a juvenile offender than on an adult offender. To ignore those circumstances would be, as the Court ruled, cruel.
The cases the Courts ruled on were Miller v. Alabama (2012) and Jackson v. Hobbs (2012).
Thanks for reading-- Max Wachtel, Ph.D.