It is completely fine for all ages (though I'm sure only kids four and up will be really interested). However, I'm not sure if anyone older than, let's just say, nine will actually care. I rarely say this about children's movies, either.
Our grandmother gave us this movie when I was 6 and my sister 4. We didn't care for it. I am now 11 and my sister 9, and our littlest 3. Our littlest loves this movie, since she loves fairies. We couldn't believe it! Thumbelina's favorite word is "um!" The main theme is arranged marriage! (which made us think about *ahem* other themes). Thumbelina sings the entire time about true love, even though she's only met the fairy prince once and is attracted to him since he is 2 inches tall. Really? And is there ONE animated movie that does not have Gilbert Gottfried in it? His appearance as the beetle was a little disturbing, since he wanted to make money by displaying Thumbelina in public at a beetle themed gathering. Ick! Not one of Mr. Gottfried's prouder performances. We gave this movie to the library once we had seen it. No thanks!