Particular interests, level of interest, and ability to read and understand a resource would be my guides independent of age. My eight year old is happy to read through an issue of Popular Science or the latest Lego Creator magazine.
I agree with Zen on this - one of my "gifted" kids read Popular Science and Scientific American and Discovery in detail at 7 years old, and one of my other "gifted" kids wanted nothing more than to read those early-reader-chapter books about Fairy Princesses (covered with rainbows… I can't remember the name of them for the life of me!).
All of my children loved Junie B Jones, Captain Underpants and various other books which were not what I dreamed of my children reading as young kids!
Have fun trying out magazines - there are so many good mags out there!
polarbear
