The results of Bandura's bobo doll study in 1961 gives evidence for banning violent games to minors. After children saw adults acting aggressively to a Bobo doll they did also. Whilst children who saw positive behaviour to the doll, didn't show any agressive behaviour. His social learning theory argues that behaviour is learnt throught observation and imitiation... therefore if a child see's violence (through whatever media), they learn to act violently.
Many other research has been done in the field of psychology finding similar results. I remember looking at a longitutional study which correlated violent video gaming time to future criminal convictions. (i cant remember the name but had to write about it in an university assignment).
So yes, I think it is right that California bans the sale of violent video games to minors. In the UK, video games are rated like films (12, 18 etc), and if a child is under the rated age they cannot buy the game.
Hope this helps.
agg, just read that you posted this question 7months ago. Oh well, better late than never!