Why do you need to wear helmet to ride a bike in California?

I'm going down to Long Beach tomorrow and I'm bringing my bike to ride around. I saw that California has a law that you need a helmet. I'm not wearing one for 2 reasons;

1) I don't own a helmet nor have time to get one.
2) I think they look ridiculous on me.

Will this get me a ticket?
Update: I know that the law is only if your under 18 and I'm 15
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