Any age, but it is up for the parents to decide whether or not their little (maybe 4/5) kids should watch it.
But, also watch out for PG-12 films. PG in one corner, 12 in the other. That means the parent decides if you have to be 12 to watch it. Also, the warnings on the back. A typical warning for a PG-12 film is moderate action violence? Some are more serious than others 9 (eg. Matilda is quite mild, containing just mild innuendo, although even that is hard to spot. On the other hand, the complete series one of Primeval contains moderate language, threat, and violence.) I must also point out that as far as I know, no PG film has no warning.