That leaves ~29 million adults. If even 10 million of those adults were either illegal or hadn't registered to vote (>1/3), that still leaves 19 million voters, not 13 million.
From the Sac Bee article excerpted above:
... There were 12.2 million votes cast in the contest between Attorney General Kamala Harris and Rep. Loretta Sanchez, compared to almost 14.1 million in the presidential contest... Turnout among all Californians eligible to vote, not just those registered, stands at 58.73 percent. That is the highest since the 59.2 percent in 2008...
The 14.1 million votes at 58.73% of eligible citizens would equal 24 million possible voters in California. The 14.1 million votes at 75% turnout of registered voters gets us to about 18.8 million registered voters. This matches your guesstimate of 19 million: I'm impressed.