The front line soldiers, especially in Europe after D-Day, had a lot of "replacements" in with them. These were the newly drafted 18-20 year olds straight out of boot camp. The experienced members of course usually held rank, even if unofficially, A Private with 6 months of combat under his belt was held in higher regard than one who just arrived.

Same thing in the Pacific theater. The ones who went in first were either career or experienced Marines and Sailors, sometimes instructors.

As an example: the case of John Basilone, an Army veteran, who left the Army to join the Marines pre-war, was awarded the Medal Of Honor while fighting in the battle of Guadalcanal, went back stateside to help sell war bonds, then became an instructor who got shipped out with one of his classes. He died on Iwo Jima aged 28, ranked Gunnery Sergeant. The men he went in with were brand new recruits.

For officers, usually a professional who was drafted got a rank somewhat equal to his stature in civilian life. Doctors were almost always Captains or Majors, as were lawyers, cops, or scientists. Teachers often got the rank of Lieutenant, as they could assist with the additional training that new recruits would need in the real world situation they were fighting, and they worked directly with smaller groups. Of course, those professional men who became officers were older and usually had more education, which raised the average of the age in combat because of the command structure.