I just recently read an article about the iPad and whether or not it is just too expensive to be giving to your child. In my opinion, especially if you go the 4G route, you are not only providing your child with a full fledged computer, but also a cell phone which is quite handy for emergencies, not to mention it has a capacity limit for data so you can pretty much guarantee they will not be over spending.. that is, so long as you keep control of the apps they may be purchasing. Of course the actual age limit would depend on just how technically apt your child may be at an early age.
From my own experience with my once-3 year old niece who was constantly sticking paper clips in to her Mom's floppy drive, because she didn't know any better.. until I bought her a 101 Dalmatians game for her to play on the computer.. within a few weeks she was flying around on that thing and didn't think about sticking paper clips in to that slot any more.
Kids are constantly texting and there are many great text free apps for the iPad as well as voice/text apps like Viber and of course Skype not to mention the FaceTime which is built in to the iPad. Add a $60 Apple Bluetooth keyboard and they have a full on computer!