Legal Question in DUI Law in California

What do you mean by "underage?" Minors who are under 18 can be ordered to juvenile hall, not adult jail.

For a first time DUI, offenders under 21 lose their license for a year, but it's only four months for someone 21 or older

Of course. Whether in juvenile court or superior court, you face potential jail time, but that is usually negotiated away in exchange for other penalties and probation by your attorney.