No, there is no exception based on length. A fixed blade of any length if concealed from view of others is illegal. Granted, if it's concealed and you are not a knucklehead who tends to attract the attention of law enforcement, nobody's gonna know.
Folding knives no matter how big (even a Cold Steel Rajah) are legal to carry concealed for you. Why not consider that.
There is no age limit on knife carry. The law applies to everyone equally.
San bernandino county and Victorville do not have any knife laws beyond those applied to switchblades (which are illegal outright for anyone under 18), so only the state law applies in your case.
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