Persons under 21 years of age in California (and persons on DUI probation in California) are subject to a very low BAC threshold for being at the legal limit: .01%. That can be a half of a beer or even less.
Not very much alcohol puts young folks at risk for significant DUI-related penalties including a minimum one year license suspension unless one’s attorney can get the DMV action set aside (no action taken against the driver’s license).
An under 21 driver also faces adult penalties, including probation, possible jail, expensive fines, vehicle impound, ignition interlock device installation, public work service program, costly alcohol programs, MADD meeting, DMV reissue fees, lawyer fees, etc. particularly if the BAC is .08% or higher.
Refusal to submit to a test is not a defense as it results in a one year suspension and is treated by California courts as an admission of DUI guilt. Subsequent refusals and DUI arrests are treated harsher with longer suspension periods.
DMV and California courts are separate & independent from each other, although sometimes a suspension action will be concurrent or nearly simultaneous.
An under 21 person must list a DUI on a college application, possibly resulting in rejection at one’s university of choice. The stigma may have far reaching implications.