All we know is that Caulifla's gang has been around for a while, Kale joined it later, Cabba is a well established soldier in the military, the girls are their age and they aren't chibi like Goten and Trunks were when they were under 17 so Caulifla, Kale and Cabba aren't under 17 either.
Her behaviour is modeled after the Sukeban, not a teenage girl. Sukeban are girl gangs filled with female delinquents. The first ones were founded by highschool girls, but that's not where the story ends. Some grew to even rival smaller Yakuza clans. Many "girl bosses" in anime are modelled after the Sukeban. It is not an age restricted thing.
They are adults