i just turned 18 and she turned 16. would it be illegal if we were to concent in sexual activity? would this be punishable by law and if so what would the punishment be? thank you for answering this question.

Under Idaho law the age at which a person can legally consent to have sexual contact is 18. It would be illegal for an 18 year old to engage in sexual activity with a 16 year old. In Idaho, this scenario is defined as "Rape"
TITLE 18
CRIMES AND PUNISHMENTS
CHAPTER 61
RAPE
18-6101. RAPE DEFINED. Rape is defined as the penetration, however
slight, of the oral, anal or vaginal opening with the perpetrator's penis
accomplished with a female under any one (1) of the following circumstances:
1. Where the female is under the age of eighteen (18) years.

The full version of this Idaho law is viewable in the answer board response titled "Idaho Age of Consent."

18-6104. PUNISHMENT FOR RAPE. Rape is punishable by imprisonment in the
state prison not less than one (1) year, and the imprisonment may be extended
to life in the discretion of the District Judge, who shall pass sentence.