TIJMKO- Most of the time in a report of child abuse their has been sufficient evidence , or a preponderance of evidence to merit it being reported to authorities. Then it's in the hands of the police and the courts to decide by going through the evidence, witnesses , etc. Children are usually very honest and unlike adults do NOT have a hidden agenda for accusing someone falsely for child abuse. Think about this : If there was not an inkling of evidence at all - then there would probably be nobody reporting any suspected child abuse .
In answer to your question : No- I would not report a 16 yr.old boy having sex with a 15 yr.old girl to authorities because it is NOT child abuse. It is 2 horny teenagers doing what comes naturally with raging hormones. It's not a crime.
I read some of your older posts in regards to your feeling guilty about having DFed people when you were an elder on hypothetical evidence in regards to an instance of a young man staying at night with a lady - but he denied anything happened. Just curious - Have you known of child abusers personally who were falsely accused ? Or do you feel perhaps badly for some in how you saw this handled in the past ? I'm just trying to understand your position and motives here