We have to at least try to rehabilitate people. Specially young people, and horrible though their crime was, we shouldn't lose sight of the fact that they are children. Of course they should be punished, but if we write people off at 15 there is no hope for any of us.
I think that at 15 they will have known that what they did was not just wrong, but dreadful. What I'm not sure about is whether at 15 people can understand what it means to be locked up for 22 years. It's longer than they've been alive. I don't think that real consequences mean much until the brain matures at 25 or so, and even then it's not straightforward. I'm sure that if we were mature at 15 we'd all have worked hard at school, studied instead of having fun, got a pension sorted out with our first pay packet, saved every spare penny, exercised twice a day and never eaten chocolate, drank wine or had unsuitable boyfriends. Before someone points out the exemplary nature of their youth, I realise that some people did do all of that, but it doesn't describe the teenage years of anyone I know. That's why there are different laws for young people.
Maybe they are beyond redemption, in which case yes, let them stay in jail, but I think the system should at least try to turn things round - both for them and for society at large.