The Ukraine conscription age is 25. If they were desperate for soldiers it would be lowered. It was 18 for the United States in the Vietnam war. Definitely wouldn't be grabbing people off the streets.

The US was a large country with normal demographics in the era of Vietnam, fighting a war with low deaths - less than 60k over ~10 years of fighting. Ukraine is a fraction of the size of the US, was already in a major demographic crisis before the war started, and is fighting a war where they are suffering high death rates.

This [1] is a population of pyramid from Ukraine 2 years ago. It's going to look dramatically worse now. They simply don't have many people below the age of 25. And you shouldn't just view those people as fodder. That is essentially the future of Ukraine. Send them off to die and you literally and figuratively send the future of Ukraine off to die.

So the two wars aren't especially comparable at all.

[1] - https://en.wikipedia.org/wiki/File:Ukraine2024.jpg