Note: I made the in-app purchase for additional sets of flash cards (for lack of a better word).
This app is easy to use for even the very youngest children. My son (currently 15 months but started using this at around one year) is not one for sitting still and loves pressing (physical) buttons (like the home button, which makes using apps for longer than two seconds tough), and this is the only app that holds his attention long enough that he has learned how to use it. It was very easy for him to learn as it requires just tapping in the middle of the screen over and over to see the doors open and close and reveal different animals, vehicles, etc.
However, there are some inaccuracies such as: an animal that looks like a duck is called a chicken, they use terms that make it obvious this app was not written by native (American) English speakers (space rocket and motor bike, for example) and the instrument claimed to be a "flute" is held in front of the body and not out to the side (a personal peeve of mine as I played the flute for a long time). I could also do without the pooping animals (they appear in some of the additional sets you get when you complete the in-app purchase in addition to the pooping horse in the barn).
My son only uses this app while siting in my lap, so I usually turn off the sound and make the noises and name the animals myself.
I think this would be great if they fixed these small issues.