Many of us can remember a bad decision we made when we were a teenager. Now imagine that mistake leads to you spending the rest of your life in prison alongside all manner of predators. As we grow older we learn the weight of our actions and to act more carefully. The idea that one moment in a persons life when they are a child should doom their existence is a travesty.
Before a person becomes an adult, their minds are not usually geared toward thinking completely rationally. Typically they do not have the mental capacity to fully comprehend the results of their actions. I think 90 years is an extremely harsh sentence for a minor. There should be more of an attempt to counsel the minor and teach them the error of their ways.
A typical life sentence is just 20 years, which is more than a lifetime in the mind of a minor. A 90-year sentence means almost certain death in prison, regardless of a person's age at the time of incarceration. Studies show that the brain isn't fully developed until about the age of 25, particularly in the area of high-order decision making, so there's still a chance for minors to grow up and make smarter choices.
Yes, a 90-year prison sentence is far too harsh for a minor. A person under the age of 18 is not fully developed yet. His or her brain is still developing, as is his or her maturity level. A prison sentence is required, but is should be shorter. Also, rehabilitation should be the goal.