Due to the fragile nature of digital devices, cost of such devices, and general lack of worry a kid shows when playing with toys, I wouldn't give one to anyone short of a High School student. Sure it would be great to give it to younger kids once they're capable of reading, but until a ruggedized device is made that can withstand a kid stabbing hard objects on the screen, beating on the device, dropping it, and carrying it around in a bag full of toys, nope, it's not gonna happen. Jr High might work, but it's just at that stage that kids tend to show more responsibility for what they have/own.
BTW If you're curious how I arrived at those examples, it's from teaching kids ranging from age 3 and up. By ruggedized, I mean something along the lines of a Leap Pad, they are pretty darn sturdy and can take a beating administered from the toughest little tykes.
Edit: Instead of making a double-post...
I really enjoyed the ideas presented by Dennis, and will keep them in mind when I have children, yet I have to beg to differ on the literacy issue posed in its relation to being read to as a child. The reason I argue this point is that none of my parents or grand-parents read to me while I was growing up, yet I took to reading with a voracious appetite. Even at this time in my life I can confidently state, and know, that my reading skills and vocabulary are well above the average individual's skill level. How did I achieve this? I'm not really too sure other than the fact that I was encouraged to read while I was a teen and I continued to do so throughout my life (you could probably chalk up the lack of cable TV to my interest in reading, 5 or 6 channels just didn't cut it
).