In the case of the U.S. Army, yes. A decline in overall manpower quality was experienced beginning sometime in mid to late 1942 as the fixed pool of men 18 to 37 and suitable for military service began to decline due to the acceleration of mobilization to its peak. Large numbers of the most physically fit and intelligent men were co-opted by the Army Air Forces beginning in February 1942, and many never reached the ground arms at all before applying for flying or ground duty training. To compensate, the dependency deferment which exempted certain men was eliminated, and physical and psychological standards for service were lowered. By late 1944, the only consistent sources of fresh manpower were men 17 years old who chose to voluntarily enlist, men turned 18 and newly registered with Selective Service, men with dependents whose deferment had been eliminated, men working in industry or agriculture for whom suitable replacements had been found, and men previously rejected but now acceptable under lowered standards. Large movements of suitable manpower also took place within the Army itself.

I previously made a post about the utilization of older men by the Army here, but I’ll repost it in its entirety below as it contains much relevant information as well as links to other answers. I’ve also written extensively on manpower in other answers on my profile here.

My follow up question would be to ask exactly the opposite. As the conflict progressed, wouldn't soldiers be more proficient, on average, as they would be more battle hardened and experienced? Sure, a certain percentage would be killed, incapacitated, captured, fled, a minority even earned (in different ways) a safer 'desk' position, and all these would be substitited by greener and, eventually, lower standard recruits; but wouldn't the majority (barring exceptional circumstances, such as much higher than normal mortality rates) just survive to become veteran? Doesn't this factor (field experience) affect performance much more than younger age / etc?