Great question. The question has a couple of factors however. First off, it depends which countries are we talking about along with some socio-economics. Note every single nation in the war will be a case study and unique.

When looking at this we need to understand that each nation needs to produce equipment and equip its troops. Troops need equipment and materials to fight. In this era and in most countries, middle aged men made up a large amount of the labor force and had training and specializations. You had miners, dockyard workers, steel plant workers ect and the most experienced men in these trades would be older. So it wouldn’t make sense to send them to war when you need their skills back in the places and trades they understand best. Not to say it didn’t happen. Just that no nation would want to do this. Also, don’t forget that Woman took over a large amount of rolls in the workforce in this era and they eventually became skilled laborers themselves, thus allowing the men to go overseas.

Let me begin with the USA. The United States is unique to your question because it is heavily industrialized, has a large literacy rate ( compared to other nations in the war ) and has a massive population. These factors play a major roll. It had a large professional workforce with middle aged men making up a large chunk of the skilled laborers in the nation. Because the US had such a large population, it did not need to rely on middle aged men to fight the war and instead could focus on younger men filling the ranks of many units. The US still had a draft and middle aged men up to 35 and eventually 37 and even as high as 65 at a point. The selective service system varied and was not set in stone as the US had a lot of men volunteering. That being said, the US was fighting a war across the world and needed men specialized in certain trades to be across the seas so you’d find middle aged men in dockyards at Ulithi and aircraft engineers in Britain, just for example.

Great Britain faced a similar situation to the US but did not have the large population reserves to fall back, at least in the British isles. It had skilled laborers and trained workers but, much like every other power, wanted middle aged men in factories and other locations. Not the frontline. This was not the case though and the UK eventually started having manpower issues when it came to filling its military. Churchill had to tell FDR around June 1944 that they couldn’t really take the forefront of the war anymore because of this issue and the US would have to.

Germany was also unique because it actually started having population issues around after the fall of France. The Germans had a philosophy of short and quick wars and this comes from cultural norms of old Prussia and also the fact that Germany does not have an empire and needs to end wars quickly before an opponent can outlast them through greater resources. German men at the times would work in factories, then go off to a campaign, then come back and work in the factories, then go campaign again. The system worked well against Poland and France but against the UK, US, and USSR they’re issues. Germany soon needed to raise the ages of conscription and eventually had to put whoever could fight in Milita units that had “old men and young boys” in them. German skilled workers were too needed elsewhere like the US but could oftentimes find themselves on the frontline.

Here are 3 examples which I hope helps answer your question. Middle aged men were generally skilled laborers and trained workers and needed in factories or mines or plants back home gathering and building resources. Some men did join who were older and did lead troops, but it varies nation to nation and even front to front. The US for example did not need to worry while the UK and Germany had to worry a bit more.