Now we’re getting into a whole different set of questions, because there are canons that deal with the minimum age for marriage. Canon 1071 goes on to forbid the marriage of minor children without parental permission. Also canon law says that the conference of bishops can establish a minimum age for marriage for each territory.
The Church will not consider an annulment unless the couple first has a civil divorce or a civil annulment. Therefore, if 2 16-year olds get married w/o parental consent (assuming they even find a priest willing to do this), but with a civil marriage license (again, assuming they can manage to pull this off), they would still have to get an annulment or divorce from the courts before the Church will consider a petition for a decree of nullity.
The Church does not want to interfere in civil law; that’s why the civil divorce must come first. In the eyes of the law, that marriage is legal, and if the Church were to declare a legal marriage to be null-and-void, that would be interfering in the authority of the state to regulate what constitutes a legal marriage contract. It would be similar to asking an ecclesiastical court to declare a real estate sale null and void based on Church law.