What are AGE LIMITS for …
From Calguns Foundation Wiki
Frequently Asked Questions: What are AGE LIMITS for …
Federal law governs FFL dealers. State law repeats that, and imposes similar restrictions on private businesses and individuals.
Ammunition
You must be 18 to buy a long gun or ammunition for it.
Handguns
You must be 21 to buy a handgun or ammunition for it, including reloading components.
If you are 18, a parent or grandparent may give you a handgun using the intrafamilial transfer rules.
Rifles, Shotguns and other Firearms
You must be 21 to buy a lower receiver which is neither a rifle nor a handgun.
Link to reference thread