My dad taught me to use his Olympus SLR when I was about 8 or 9 and he got me one of my own when I was 11. That's how I got started. I still have my dad's old camera, he gave it to me when he bought a digital camera. My first camera got damaged in a car accident so sadly I no longer have it.
When my son, who is now 11, was little he wanted to mess with my cameras, looking through the viewfinders, tripping the shutters, etc. He was only 3, so I got him a Fuji Quicksnap (one time use camera) and he was excited about it. He took a photo of me and then I told him to let me have it back so I could show him how to wind it and how to turn the flash on. He held it close so I couldn't take it and said "I know how to do it!". He wound the film himself but then puzzled over the flash (you have to charge it before each shot by holding down a button on the one I had gotten him). I showed him how to do it and he finished the roll of film in it all by himself. We got it developed and he had some GOOD stuff! His compositions were very good, especially since he was only 3 yrs old.
He used a bunch of those single-use cameras until I got him a 35mm auto point-n-shoot when he was 4. The cost of the Quicksnaps was too high compared to buying 4-packs of cheap 35mm color film. He used it for several years until his mother, who was also a photographer, bought him a digital camera that he still uses.
My son isn't into photography as art or as a hobby in itself like I was as a kid, he just uses it to document things he does mostly, and he photographs our cat and the family. He's been active on the forums on lego.com and he photographs things he builds with Legos to post there. He takes a lot of photos though and is very good.