The CPS has a two part test which is undertaken when deciding whether or nor to prosecute an alleged offender. The first is the evidential test which says (briefly) that prosecutors must be satisfied that there is sufficient evidence to provide a realistic prospect of conviction. This does not seem to be an issue here.
However, the second part is somewhat more subjective and that is the “public interest“ test. To be brief again, this says that in every case where there is sufficient evidence to justify a prosecution, prosecutors must go on to consider whether a prosecution is required in the public interest. There are many facets to this test and the entire code can be found here:
http://www.cps.gov.uk/publications/docs/code_2013_accessible_english.pdf
Of particular relevance in your case would be paragraph 4.12 (d). To save you looking it up this says:
“The best interests and welfare of the child or young person must be considered including whether a prosecution is likely to have an adverse impact on his or her future prospects that is disproportionate to the seriousness of the offending. Prosecutors must have regard to the principal aim of the youth justice system which is to prevent offending by children and young people. Prosecutors must also have regard to the obligations arising under the United Nations 1989 Convention on the Rights of the Child. As a starting point, the younger the suspect, the less likely it is that a prosecution is required.”
It seems likely that this clause is the one that has swayed the decision and a review of that decision under the guidance suggested by hc, would be the only way forward. You can see that a balance has to be derived bearing in mind the seriousness of the offence as well as other factors. A chat with a solicitor may help you understand how the CPS operates when reaching these decisions.