Well the clue's in the publishing specification surely - they're for young adults.
Some would argue that this is in itself patronising, but the inference is there - the books may be written about children but it's for those on their way to maturity... That covers soooo much possibility it's untrue, no matter how old you are
However, WFM keeps getting a mention and just with how that starts with Jenny Greenteeth and the frying pan, Tiffany is not by any definition a 'young' girl. What would appeal to me if I were a 'young adult' still is that there's hardly a whiff of dumbing down in the series - its whole environment's gritty as befits a rural/farming community where the kids work their passage as much as the grown-ups as it would have been in compatible times in the crofts of Scotland say (or out in antipodes
) at the turn of the 19th century with travelling teachers and children brought up to help out in the dairies etc. In Wintersmith and the 2 previous books Tiffany's 'right' for the age she's depicted at if a little solemn and more serious than other children because of her inherited talents with her granny certainly making a huge impression throughout and the echoes of that run through all her books with Roland too in terms of legacy and continuity which are important to kids as they grow up in those close-knit communities. I think this is also why we like the Witches so much because they are the lynchpins of the Chalk and Ramtops communities and Tiffany's journey, clever and exceptional as she is, is still simply a part of the cycle.