My other grandfather, who is technically my mother's step-father but the only grandfather I remember, also turned 18 on D-Day. I don't think he even made it out of the state during WWII although he was in the Army for a short time. I always envied people who had stories of their grandfathers' WWII service.
My father was a career Air Force officer and served in Vietnam and the Persian Gulf. He retired after 20 odd years of service. Recounting his service would take more of a book than a blog post.
My husband's family is a different story. His Papa Mark (photo below) served in the Navy during WWII on the USS Haas and USS Hilary P. Jones. He had some adventures and clearly enjoyed himself. I loved listening to his stories of his service every Christmas.