Keep in mind that unless you get the New Millennium editions, the first three books were written in the 80s/early 90s, so they are quite dated. I gave the first book to my oldest when he was 13 and he came up to me to ask what a dictaphone was. However, she's updated them fairly recently so that modern teens aren't thrown out of the fantasy by terms they're not familiar with.
Regardless, they are really fabulous. The magic is much more scientific than HP magic, and their goals are lofty and worthwhile.