it is not a given that sentencing guidelines need to be taken into account from back then, or that they should be sentenced as juvaniles. This had afaik been a request made by their lawyers (according to someone on BBC, and tbh I'm more inclined to go with them than sky). Also, only one of them was seventeen at the time, the other was eighteen (I can't remember which) so even if this ends up being the case this would only apply to one.
Also, it is not a maximum of twelve years, twelve years is the minimum tarif that can be served, but as the crime is murder it is likely that they would be detained at her majesty's pleasure with a minimum term of twelve years at which point they would be eligible for parole, doesn't mean they'll get it though.
Norris had already been tried for the attempted murder of someone else, but was found not guilty due to bribery and corruption allegations against the foreman of the jury, and Dobson was serving a sentence for drug trafficking at the time he was being tried for this crime.
They're not exactly fine upstanding members of society.. 