I think it is age appropriate for a 12 year old.
There is a lot of subjects covered in this movie (sex, nudity, illness, war, child abuse, discrimination, racism, drug abuse, alchoholism) just to name a few, however it is all discreetly done. Almost nothing is outright shown on the screen, it is mostly alluded too so a lot may go over a 12 year olds head. But I think that a lot can be learned and understood from this movie because of the way things are explained (it is kind of through Forrest's eyes, and he probably has about the same mentality of a 12 year old).
However, that being said, I would probably still recommend you viewing the movie first and making the decision for your child.