I rate this at a later age for these reasons:
Speed traps: You are encouraged to drive as fast as you can past a speed camera. Until a child understands this is a game, and you don't do this in real life that is an issue.
Road Racing: The real world driving is great, but you are racing on roads. It keeps track of your average speed. This would encourage you to speed faster to raise this. There are other no racing cars on the road. No real damage is done, and no one gets hurt. This concept needs to be understood by the child before introduction to this game.
Wreckage: You get points for causing wreckage. This seems innocent, but you need to be aware of your child's understanding that this is a game. If your child doesn't have a firm grasp on reality versus make believe, that this could be an issue. In real life there are consequences for running over signs. You don't get discounts as you do in the game.
Consumerism: While I don't find this an issue, it is there. Be aware. If you find this to be a bad thing, than make sure you pay attention to the fact that it is in the game.
Sex: I listed this as there are some skimpy outfits. This is based on your values. I can see this being suitable for a child younger, but you need to be aware of your child and what they are used to seeing. Like it or not, it probably isn't worse than they will see on "Dancing with the Stars".
Violence: This falls into the wreckage I mentioned above. When you view your car for a photo op, you can see the damage done. It is cosmetic, so there is no negative to having your car all smashed up. Make sure your child understands this concept.
Safety & Privacy: If you child has Gold, they can play online. If you like me do not allow your child to have Gold, there isn't a concern.
I know others have mentioned language, drinking, drugs, & smoking. I could have missed them honestly, but don't remember seeing them. For the language, I'm sure you would have to look at the entire soundtrack to see what is in it. I heard house music the most, so be aware. I know house music can reference drugs. I don't remember hearing it, but found out "looking for molly" isn't looking for a girl in other media. Just be aware of slang terms for this in the music.
I hope this helps. Just be aware that it seams innocent on the surface, but there are a lot of things that push this title to a higher level. Unlike the other Forza, witch focus on track racing (or closed road), and has negative effects to the car if you crash; this one is more lighthearted. In being this way, more negative can be derived.