They sure are, and I don't really think there is a minimum age at which newborns and beyond should be allowed access to them. Yet it is of utter importance they also get access to actual non-flat toys/tools/... in order to develop dexterity, the feeling to handle objects and even creativity. Also not to forget about getting enough movement/excercise and not getting used to a sedentary lifestyle: most tablet apps unfortunately don't require the user to move around, on the contrary.
And as for screen time I have no regrets about letting her play various alphabet games on the ipad* to keep her stationary on the potty during toilet training.
It was weird realizing that she might never buy a keyboard.
*along with books toys and anything that would keep her there
My kids were all very comfortable with mice at a very young age, not as much typing until they needed to type to get things done. (it always annoyed me that the Reader Rabbit app had the same icon for the uninstall and launch feature though.)
Next up will be better audio processing and then your two or three year old will able to say "Why!" repeatedly to your tablet and get an endless stream of entertainment.
For an 8 or 9 year-old, that is fine. And navigating a file/folder system can be quite an adventure. But some of the older ones (10-12) lately are having trouble with finding the proper X (close) button when there's multiple windows (and sub-windows) on the screen. Or what minimizing does (the window is gone but it's not really gone? how will you figure out the restore button?). Definitely a tricky one is what the taskbar is for (most tablets / phones don't have that).
They do love it when I teach them "special tricks" like copy-paste with shortcuts, moving by word with ctrl-arrow. What NumLock does. And for some reason they all know "backspace", but do not discover the "delete" key. One kid, upon explaining it's like backspace but deletes the other way: "oh my god, this is so cool I'm going to use this EVERYWHERE!!!" (and I smiled and thought, yes, DELETE ALL THE THIGNS)
:-)
[0] I teach children at a volunteer's centre to do interesting/creative/technical/cool stuff with computers--anything that gets them enthusiastic that isn't YouTube or Minecraft--yes yes Minecraft is a very creative environment in a sense, but it's quite easy for them to revert to "consuming" all day long instead of "building/crafting". However I'm also aware that this is my own lack of knowledge, meaning I can't properly stimulate them to take on creative challenges. Plus they can't play (properly) without their own paid account (right?), making it a bit unfair towards kids whose parents can't or won't buy them an account (the volunteer centre itself aims to be almost-absurdly cheap, €20/year, attracting children from all strata of society).