Until they're less expensive and fragile, early teens (12/13) for me. And I say this having given my elder son a PC on his fifth birthday (he was using mine too much! lol) and I allow both him and his youger brother (now 4 1/2) to play a little on the iLiad (they love drawing on it, and the 9-yr.old has done a few Sudokus). I think the flexible displays that are being developed will probably be great for little ones.
Actually, if the Sony were available here in Spain, with the 50$ offer, I might have considered it for my eldest.