Does it sound sensible to write off a teenager because of two serious convictions that don't include murder? Not to our mind.
As a matter of general policy, there are reasons to be skeptical about jailing a youthful criminal for the rest of his life. Juveniles lack the judgment and impulse control to be fully responsible for their actions, and they may be more amenable to rehabilitation than adults. With 77 inmates serving life without parole for non-homicide crimes committed as juveniles -- twice as many as all other states combined -- Florida clearly puts excessive reliance on this option.
While they may be more amenable to some new, untried rehabilitation that we have not been practicing, the percent of released prisoners from all states who were rearrested within three years is a monotonically decreasing function. The older you are, the less likely you are to be re-arrested, and re-convicted. It seems to be in our interest to put younger individuals away longer than older individuals.
Additionally, of all violent offenses, property offenses, drug offenses, public order defenses, and other offenses, murderers are the least likely to be re-arrested or re-convicted within three years Releasing the marginal murderer seems much less criminal than the marginal rapist using recidivism statistics.