Depending on when the relations started different timelines can be at issue. I believe the individual has six years after their 18th birthday to report the offense.
However, there is an affirmative defense of downward dating if the individuals involved in the sexual relations are within three years of age of one another.
If this does rear its ugly head feel free to contact me because sex crimes are very delicate cases that a lot of lawyers don't know how to properly handle.
Platten Law Office
In a technical sense, that would be "sexual assault." The girl would have an affirmative defense since they are not more than 3 apart in age. Any reasonable person would have to question the motives of the mother as well. It is almost inconceivable that a teenage girl could "rape" a 16 year old boy. There is a point where the prosecutor and the police would not waste their time on a ridiculous allegation.
This of course assumes the sex was with physical consent.
The mother would have to be a real witch to force two teenagers into the criminal justice system for doing what teenagers do. It sounds like the mother has a personal conflict with the girl and wants to use the system to attack her. If that is the case, that is pathetic.
If there is more than 3 years difference between the two, any sex (consentual or not) is punishable by up to 20 years in prison.
The 19 year old would be wise to stay away from the 16 year old with the upset mother.