If it were a matter of maturity, I would have given an age recommendation of 10 and up and possibly even lower. Mature content may not be an issue but the player must be old enough to truly appreciate the art and themes of Machinarium and must have the knowledge to solve these rather difficult puzzles. It's worth playing, but I'd give this an iffy rating for younger kids because they may not appreciate the level of difficulty of the puzzles.
As far as mature content goes, there isn't too much to find here. There are a couple small references to violence, but it is mostly the type you would see in playground bullying. Josef, the main character, is bullied by other robots and memories are shown of him being pushed, having money stolen, having a sand castle destroyed, and other bullying acts. Residents of this city you explore have also been effected by this bullying. One person had his radio stolen while another had his drum broken, showing the negative impact on bullying. The only other violence that is depicted in the game is a bomb threat. The bullies strap a bomb onto a tower where Josef's friends are being held. An animation shows the tower being blown up from a distance, however you are able to rewind time in order to save your friends.
Other than that, the only other issue parents may have is there are two scenes where drugs are used. One is in an earlier level where Josef is held in a prison cell and must make a cigarette for an inmate in order for him to help Josef. To make the cigarette, he finds a some wet weeds in a pipe, dries them on a lightbulb, and then wraps the burned weed in toilet paper. Additionaly, Josef has a a flashback of making a cigar with the end of a cattail plant, but quickly hides it when a guard comes near. Both creation processes are unrealistic, however it may give kids ideas.
This is the only mature content seen in game, however I raised the age recomendation to 12 and up because the puzzles can be excruciatingly difficult. The puzzle elements can be hidden a little too well in the environments and they can really fry your brain. Fortunately there is an in game walkthrough for all the stages, but it would have been nicer if this was not necessary if the puzzle difficulty was toned down a bit.
Despite the puzzles being difficult, I found it definitely worth the ride. The music and art work was amazingly done, the characters are a delight even without speaking a single word, and the game just makes you feel like a good person. Josef is not solely focused on saving his friends, but he additionaly helps others who have been affected by bullying along the way. Additionaly there is a cute love story between Josef and his girlfriend, one of his friends he is trying to save in the tower. Josef is a great rolemodel, and his character really makes up for the game's difficulty.