What is the age to purchase an assault rifle in ca?

An actual assault rifle is a select fire (full auto or burst) firearm - you'll never get approved for an NFA permit in California.

California has had a so-called "assault weapons" ban on civilian semi automatic firearms since 1989. You can still get the same rifles, for the most part, but they have to be California compliant. The receiver must be from a manufacturer not on the California Department of "Justice"'s list of banned models and manufacturers, and the rifle either must have a "bullet button" installed (thus making it a fixed magazine rifle under California law), or it must be featureless, meaning it can't have any of the following features:

• Flash suppressor
• Pistol grip
• Forward grip
• Collapsible or folding buttstock

You can buy them at 18, same as any other state.