I suppose if you have $499- $699 easily disposable then any age 6 and up is ok. Because children throw temper tantrums and break things that have glass easily, I could see a child throwing an iPad on the hard floor, or banging it hard on a table when they get angry maybe. I think around 8- 10 children have an appreciation for expensive electronics and that may be a good age to get them their own.
I think it's interesting that children are able to use it as a learning tool, and I agree that it does have some benefits as children in the future will probably have to be more and more tech savy. As for reading, all the book stores are disappearing and books may all be digital someday, not to mention that's a parenting issue, regulating your kids play time.
However many studies have shown that children are developing differently then they have been and believe that some attention problems may be due to focusing on computer video games and all kinds of other things no one here would really want to admit. As well as children even ages 4 and up lack certain fine motor skills from not using crayons and pencils unlike children that use utensils and pencils have had for over a few thousand years.
My main concern would be a child breaking it, and being out a few hundred dollars.