Closing this. Please read over our forum guidelines before creating any new posts OP. The legal side of things is definitely not something we handle here. In the age of information that we live in, it seems pretty criminal that you can't just Google, California+Tobacco+Laws. You don't even need to click any links, Google will display the answer for you in the search results as if you were asking a member of the legislature at his or her own desk.
The age of purchase in California is actually 21 years old since, I believe, June of 2016. While I'm sure the law is fairly important, things like "underage use/possession of Tobacco products" has been a slap on the wrist/up to the discretion of law enforcement. In general, police do not walk up to people and card them if they're suspicious of their age. If you get caught, it could be that the cops stomp on your cigarettes, call mom and dad or have you appear in court. Like I said, it's really discretionary in a similar fashion to underage drinking.
I was shackled in court one time for possession and their was a teenage girl their who was made to read an essay she wrote on the dangers of smoking to the entire court. I'm sure it varies.