It is quite straightforward really.

The writer has chosen "des" for a reason.

Had he chosen "aux", he would have been referring to all young people (aux = to the) between the age limits defined. As he has opted for "des" he means some of those young people within the age limits defined.

Thus the book, he believes, will be accessible to some young people within that age group, not every single 12-18 year old.

A quick word on "accessible". WHen the writer says that the book is accessible, he does not mean "available". He means accessible in the sense of "readable" and "able to be understood by". This ties in with what your instinct is telling you about "aux" and "des".

That said, whether in English (GB) we would in this particular instance go so far as making the difference which in the (French) writer's mind no doubt makes all the difference, is another matter.

I would never the less opt for saying "a book on the Haulocaust" has become accessible to young people between 12 and 18 ...", defending this option in saying that in modern English, in context, it is obvious that only some of the specific group are concerned. The "some" is understood and thus not required.