My condolences.
Both of my grandfathers were in their early 20's when they went of to WWII (DOB's: D.M. "Bud" Gray in 1917 and R. W. Bennett in 1919), if your late g'pa was born in 1928, he went off to war at the age of 15-17??? I know some of the guys who went ashore in June 1944 were younger than 18, but I was under the impression that as the way went on the enlistment requirements were more stringent. Either way, kudos to him and his service.