This question is actually more for my friend rather than me, as I will be 18 for both the general and primary elections. He and another of my friends, on the other hand, won't be 18 during the primaries, but will be 18 during the general elections. So can they vote in the primaries?
No. The current California constitution does not include language to allow this.
However, there was a bill introduced to the state Senate to amend the constitution to allow this. If passed, it would allow 17 year olds who will be 18 on the day of the next general election to vote in both primaries and special elections:
That Section 2 of Article II thereof is amended to read:
SEC. 2. (a) A United States citizen who is at least 18 years of age and a resident in this State may vote.
(b) A United States citizen who is 17 years of age, is a resident in this State, and will be at least 18 years of age at the time of the next general election may vote in any intervening primary or special election that occurs before the next general election.
Italics indicate added text
For this to take effect, it must first be passed by 2/3 vote in both chambers of the state legislature. Then it will be placed on the ballot during the next election (in 2016, most likely).
Of course, by the time it takes effect (if it does), your friends will not be 17 any more, so it won't affect them.