If a 17 year old buys a youth license and tags at the beginning of the year but turns 18 before the season starts can they still shoot a doe in the youth hunt units?

No. Youth hunts are based on the hunters age and not the license or tag type they possess. According to Department rules, "Youth hunt only: Some deer hunts are for youth only. Only hunters 12-17 years of age with a valid license and tag may hunt antlerless deer in these units". Those units are listed in the pages with the season information. This information is found in our big game proclamation in the deer season section.