If I am 15 years old, but I will turn 16 within the license year, do I buy a youth hunting license or an adult hunting license?

While you are 15, you may purchase a youth license and use it for the entire license year. You will need to buy an adult hunting license and appropriate tags and state migratory bird validation to hunt waterfowl in subsequent years, however.
Licenses may be purchased online or at authorized license vendors.
If you plan to hunt waterfowl after you turn 16, you first must purchase a federal migratory bird stamp. All hunters 16 and over must possess a federal migratory bird stamp signed in ink across the front. Federal stamps may be purchased at selected license dealers, all post offices, all National Wildlife Refuges, by calling 1-800-STAMP24, or ordered online at www.duckstamp.com