Classification categories

The age classification categories are as follows (according to the BBFC):
U - Suitable for all
PG - Parental guidance
12A - Cinema release suitable for ages 12 and over
12 - Video release suitable for ages 12 and over
15 - Suitable only for 15 years and over
18 - Suitable only for 18 years and over

Here are some comic book films and their BBFC ratings:
Superman Returns: 12
Batman Begins: 12
Hulk: 12
Scott Pilgrim Vs The World: 12
The Avengers: 12A
Catwoman: 12
Wonder Woman: PG

The most popular comic book films were all rated 12, according to the BBFC, this is the description of the rating 12:

The BBFC's Guidelines state that there may be strong language (eg 'f***') at 12 or 12A, but it must be infrequent. The context of the strong language is important. Aggressive uses of strong language may result in a film or DVD being placed at the 15 category. There is some allowance for puns on strong language at this category.
There may be moderate language (eg uses of terms such as ‘bitch’ and ‘twat’ at 12 or 12A).
Any discriminatory language will not be endorsed by the work as a whole. Aggressive discriminatory language (for example homophobic or racist terms) is unlikely to be passed at 12 or 12A unless it is clearly condemned.

From this, we can conclude that most comic book films may have some strong language, so they are not suitable for young children, however, the language won't be frequent and therefore can be viewed by people 12 years and up.