They will be held up to 21. They can easily be released sooner than that, and it's based entirely on whether they're considered "rehabilitated" or not, with punishment for their crime having fuck all to do w/ it. Because there isn't a long and documented history of repeat offenders and high recidivism rate, so we should trust the authorities to know when they're no longer a danger to society. And no, we'll probably never learn their identities, even after they turn 18.
Meanwhile, out of curiosity, I googled, "15 year old jailed for" and got results like these:
https://en.wikipedia.org/wiki/Lionel_Tate He was 12, and got life in prison. But yeah, expecting the suspects in this case to even spend 5 years in adult prison is asking too much. (He was eventually released, and went on to become a fine, upstanding citizen...psych!)
Mind you, I don't agree with all of the above actions/sentences, just using them as examples to highlight how FUCKING RIDICULOUS this plea deal is by comparison.
...you're aware that you're comparing AMERICAN cases to a British case, right? It's particularly worth noting because America's "justice" system is arguably the most brutal in the developed world (and, I should mention, has the world's highest per capita incarceration rate -- largely because of the ridiculous length of its sentences).
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(\/)(\/)|-| There are precious few at ease / With moral ambiguities / So we act as though they don't exist.