The legal system allows an individual who was under eighteen at the time of conviction to apply for release when half the sentence has been served.
However, if you read the link, it advises that such a release is extremely rare, so there is absolutely no guarantee that Mr Davies is going to have his appeal granted - and the advice of the link points to the indication that he is likely to be in prison for some time yet.
A further thought - the right to appeal after half the sentence has been served is mandatory, and is not affected by the prisoner's status.
His status may influence the powers-that-be with regard to his suitability for release, but again, given the circumstances of the crime, and his evidenced persona at the time, I would suggest there is little to indicate that a sympathetic response will be forthcoming.
Please note he has only APPLIED for release . That DOES NOT mean he will be successful . As far as I have been able to ascertain he has little hope of success. Prisoners apply for early release because they have a right to on a sentence of over 5 (I think) years. Applying for release means getting out of your cell, into a court hearing.That is a lot less boring than sitting in your cell 23 hours a day. (1/2 hour release for lunch and another for evening meal) I have been in prison as I have reported on here, believe me it is BORING !!!!!