In your example the 18 year old is an adult the 14-16 year old is not.
Interesting thing to consider here. If a person is one day shy of 18, and their sexual partner goes one day over, is that rape?
For the record, it is and has happened. A good friends son spent 5 years in prison for this exact "infraction".
The parents of the young lady didn't like him, and hired a lawyer to check out the legal path that they would follow.
The day he hit 18 they had him arrested and charged. Tx prisons are not fun.