I notice that looking at old ww2 pictures, a lot of them are young and hold the ranks of E-6 or E-7 hell even some of them have E-8. Today, very rarely do you see a young e-7 or e-8, even e-6's can be in their late 20's early 30's. Was it because there was no time requirement to rank up in the army during ww2? Or was it just wartime rank-blazing?
So certainly, the massive expansion of manpower requirements played a huge part in this, but there is one aspect which ought to be focused on here, namely that if you were a Officer of the Peacetime Army prior to the beginning of the draft in 1940, you actually held two ranks, one in the Regular Army which was your real ("Permanent") rank, and one in the "Army of the United States", which was the term for the wartime entity created by conscription in 1940 as the US began to think about the possibility of becoming involved in the conflict. This latter rank was your functional one, or "Temporary Rank". You're certainly correct that this meant young men would often hold ranks far beyond what you might expect for their age, but for those who were also members of the Regular Army, their "real" rank might lag behind considerably.
To use one famous example, George S. Patton was promoted to Colonel in the Regular Army on July 24, 1938. When the draft began in 1940, many officers saw an immediate bump to their rank in the Army of the United States, and he was no exception, getting his first star October 2, 1940. But he was still a Colonel in the Regular Army, and would remain so until August 16, 1944, by which point he was already a Lt. (Three Star) Gen. in the Army of the United States. He was given a double (Regular Army) promotion, straight to Maj. (Two Star) General at that point, and his Regular Army rank finally caught up with him in December when he became officially a Lt. General. Having been longstanding friends with Eisenhower, and somewhat resenting the fact Ike was now his superior, it was a minor point of pride to Patton that until Ike's promotion in 1943, despite outranking him in the Army of the United States, Patton still outranked him on the Regular Army rolls for a time.
So anyways, that is essentially the sum of it. The Wartime military used separate ranks, and advancement there could be insanely quick.
Side note, this is also true in earlier wars, although the names for the force weren't always the same ("National Army" in WWI for instance), and perhaps the most famous example is George Armstrong Custer, often remembered to posterity as "General Custer" as he was a Brevet Maj. Gen during the Civil War, but who in fact died a Lt. Col. after reverting to the peacetime ranks after the war.
A lot of it was attrition, and the sudden expansion and continued formation of new units needing new leadership, and battlefield losses, but also some of it was the transition from small peacetime to war footing.
The small prewar army meant that men who were older than they ought to be were stuck without promotion, and when war came we see a whiplash to the other direction.
As an example for the Navy in the early going in the first quarter of 1942 every officer below Commander(O-5) who had been in service prior to Pearl Harbor was given an automatic 1 rank promotion. Thereby better reflecting their actual seniority and duties.