There's a big difference between having older soldiers fighting a defensive war on home soil vs sending them overseas.

In the first scenario, you desperately need a lot of warm bodies, most of what these people would be otherwise doing has been shut down, if someone does perform a critical role in society at large, going back and forth is quick, and the alternative still potentially leads to you losing that person.

In the second scenario, recruiting middle aged people robs your economic/industrial/cultural base of its experience and mid-level leaders who are critical to stuff getting done. Substantial resources are spent training, moving, and sustaining these troops who are not as well suited as younger individuals, on top of the opportunity cost. Besides the people already in the military who have spent years gaining applicable military experience, those additional bodies are liabilities, not assets. An argument can be made for raising mandatory retirement age to keep those skills around, but not for new recruits.