I volunteer in a school, and my main job is to encourage children to read for pleasure, particularly boys. Many of the boys in the older classes (primary school so ten and eleven year olds) are obsessed by the Alex Rider books, of which this, Stormbreaker, is the first in the series. I decided that I should read it so that I could get to grips with the kind of thing they enjoy and meet them where they're at, rather than expecting them to start with Pride and Prejudice.
I have to say that I thoroughly enjoyed this book. It is plot and action heavy and the pace is fast. The chapters are short and punchy and there isn't a great deal of philosophy or serious emotional stuff going on here. The book starts with the assassination of Alex's uncle, the man who has been his guardian since he was a baby, and Alex copes with his loss remarkably well. The story just speeds forward onto the next action sequence and the next. It is written very much like a computer game, one problem gets solved, the next one immediately springs up, and it is generally one which requires a great deal of physical action and not a terrible amount of anything else.
It was silly, and implausible, and at times, my rather more rational grown up head just wanted to shout 'no! that's rubbish!' but the part of me that just appreciates a very silly action romp shouted it down.
It is a relatively quick read, which I think is also an enormous part of the appeal.
I would say ten is probably the youngest I would recommend this too unless you have a particularly precocious reader on your hands. There is a lot of violence, a lot of casual death and some fairly gruesome events which may not sit well with younger readers/listeners.
If you are looking for something if your child has read the Alex Rider books I would suggest trying Charlie Higson's young James Bond series which are the next level up, slightly wordier, slightly more thought provoking, but just as exciting.