Arizona Law says (ARS §13-1405) that it is generally illegal for anyone over 18 to have sex with a person under the age of eighteen. There are exceptions, however. These exceptions can be found in specific Arizona Laws (See ARS §13-1407(F) -- which sets out a relevant defense to the sexual offense in ARS §13-1405)

If all of the following are true, then it is not criminal for two teenagers to have sexual relations:

1. the minor (or the younger person) is at least 15 years old
2. the older person is less than 24 months older than the younger one
3. the activity is consensual
4. the older person is under 19 years of age or still in high school

Therefore, a 16 year old can have sexual relations with an eighteen year old as long as the activity is consensual and the 18 year old is less than 24 months older than the 16 year old. If the 16 year old turned 16 in July of 2001, and the eighteen year old turned 18 in June of 2001, they are 25 months apart in age and it is illegal.