The 26th Amendment to the United States Constitution provides: "The right of citizens of the United States, who are 18 years of age or older, to vote, shall not be denied or abridged by the United States or any state on account of age".  The amendment, however, says nothing about minors voting corporate shares.  The California General Corporation Law, however, does. Section 702(d) of the Corporations Code provides: 

"Shares standing in the name of a minor may be voted and the corporation may treat all rights incident thereto as exercisable by the minor, in person or by proxy, whether or not the corporation has notice, actual or constructive, of the nonage, unless a guardian of the minor’s property has been appointed and written notice of such appointment given to the corporation."

The effect of this provision is that shares held in the name of a minor may be voted by either the minor or the minor's guardian, depending on whether written notice of the guardianship has been given to the corporation.  Note that Section 702(a) creates an exception to the general principle that holders of record are entitled to vote.  If a guardian holds the shares but the shares have not been transferred into the guardian's name, the guardian may nevertheless vote if written notice is given to the corporation.