It would make it so that Finn wanted to join the First Order, which in and of itself wouldn’t make sense.
I think it could be possible. We know he was a janitor for most of his First Order career, and the raid on the Jakku village was his first battle. It’s possible that he just joined the lowest ranks of the First Order to make a living, but upon seeing the atrocities that the First Order committed he had a change of heart. The other troopers don’t have that excuse, they’ve presumably been beating down and oppressing people for a long time.
And brainwashing can happen at any age, either way there would be a level of brainwashing happening. I feel like we’re just looking for excuses to not care about people dying, which… I dunno. Kinda goes against the “empathy” argument to me.
Well, people tend to be less receptive to brainwashing at an older age. It’s why so many authoritarian governments turn the school system into a propaganda machine: they want to make their citizens into brainwashed servants at the youngest age possible. If the stormtroopers were voluntary adult recruits, that would mean they agreed with the First Order’s neo-Imperial ideas at least somewhat. And I don’t think getting revenge on oppressive mass murderers signifies a lack of empathy, if anything it signals the opposite: you can recognize when someone is being oppressed, and will do whatever it takes to stop it.