I say that this is an appropriate movie for approximately the age of thirteen and up, though I myself watched this as a fifteen year old. It's beautifully done, with a hauntingly memorable and entrancing story line and soundtrack, especially L'exécution, with it's almost merry-go-round reminiscent style.
The main visual, background and costume aspects are Steampunk, a style which many would know of from such famous writers as H. G. Wells and Jules Verne, with mechanical apparatuses being common use.
It definitely is not a film for the majority of younger children (mainly under 12s), seeing as the storyline does have the possibility of being hard to follow for younger kids, not to mention that there is a fair bit of bad language, and there are many nightmarish themes which may frighten them. But for more mature children and thirteens and up I do recommend this film- it may not have been a blockbuster, but the storyline is heart-wrenching, the characters are believable and the ending is gorgeous but creepy. Well worth it.