Update: Initially, we wrote that "at the time, the Yeater was 19 and Bieber was 16, so under California law the alleged act was not statutory rape." But, unsurprisingly, we were not quite correct. A defense lawyer in California tells us:
Penal Code Section 261.5(b) makes sex between anyone over 18 and a minor under 18 considered "statutory rape". A minor cannot consent to sex, period, under California law. Thus, under PC 261.5(b), this alleged mother is at least guilty of a misdemeanor. If there was over a 3 year difference, it can be a felony also. She would not have to register as a sex offender upon conviction, however.