I feel that some of you here don't really know and understand Black Mask. In the comics he's a wealthy heir to a cosmetics fortune (who ended up murdering his parents, with the complicity of his model girlfriend, 'Circe'), and a contemporary of Bruce Wayne. He's portrayed as a rather charmless, nondescript type whose sociopathic personality only comes alive when he assumes his Black Mask persona.
In many ways he's like the evil mirror image of Bruce Wayne, and so it would make sense to cast an actor who is roughly Ben Affleck's age and of similar temperament. Personally, I think Matt Damon, (Ben's old mate) would be perfect in the part, or, if he's now willing to play a comic-book character, having finally won an Oscar, Leonardo DiCaprio.
But he should certainly be portrayed as a blue-blooded, old-money type who chose to enter the criminal underworld rather than someone who was born into the gangster lifestyle.