This is called a children's book by many reviewers, and indeed by many newspaper best seller lists. If you want to call older teenagers "children" then, despite the dangers of such a description, you could be right. I would not offer it to a younger child for it has some dark moments, especially in the opening chapter which deals with a teenager who loses her unborn child after a brutal beating from her father which her mother did nothing to stop. This real violence, as opposed to the joke violence of earlier Discworld novels,is a new departure for Terry Pratchett but it is simply an indication of his growing maturity as a writer. It is certainly a book that can be fully enjoyed by adults. It is a fully fledged Discworld novel and many old friends make cameo appearances. Yes, there is also a good stock of one liners.
It is disappointing to read in some reviews that there is no character development. What on earth do these reviewers want? Have they ever read a novel? Quite a number of minor characters do develop markedly and not by magic but by facing up to difficult situations. Not just minor characters, but Tiffany Aching continues her development which was such a great feature of Wintersmith. She faces the type of challenge that helps to make us mature adults and develops as a consequence. I don't think I'm giving much away when I say that she does hardly anything we might call magic; that is anything that might be at home in the Unseen University. How she resolves situations, or at least comes to terms with them, really is magical.
Finally you experts on Sir Terry's Alzheimer's disease who can delicately discern his deterioration: you know nothing about the condition other than the usual prejudices. The condition is not all negative and may be instrumental in helping Sir Terry to branch out in his writing. As this development occurs enjoy what you have been given. As the author knows full well, it won't last forever.