It seems pretty clear to me. I am a layman not a lawyer, so what do I know?
To me, what the Supreme Court said was that life without parole cannot be automatically applied to minors. If the state where the minor was convicted allows for life without parole for a specific crime, a minor cannot automatically receive that sentence.
It does not say that minors cannot receive a life without parole sentence. I believe that it means that a judge or jury must consider all factors pertaining to the specific minor before imposing a life without parole sentence.
So far as the role of life expectancy in sentencing, my impression is that it is not considered a factor because no one really knows how long someone else will live. And no one really knows what medical advances will happen to drastically prolong life expectancy.
That's interesting. My interpretation was that if a youth commits a crime, no sentence that excludes a reasonable parole date can be given. It's also my understanding that a youth cannot be sentenced such that he or she spends the remainder of his/her life in prison.