In the Star Wars sequels, we find that the First Order is filling their ranks with slave labor conditioned from early childhood for loyalty. Is this the only way to become a First Order Trooper, or is it possible for adults, or at least older children, to voluntarily choose to enlist despite not being raised in the First Order's childhood regime? Reason is not relevant - maybe someone really hates the New Republic, maybe they think fighting for the First Order would be a stable job, maybe they just like guns and glory and are willing to fight for anyone if given the opportunity, etc. Is such a wannabe SOL, or is there a process for them to prove themselves and put on the helmet?
I am relatively certain that you can enlist. You could enlist in the Empire, and the First Order has a lot of propoganda as well that they wouldn't use if they didn't want soldiers to enlist.
Also, I do not think that Captain Phasma and other high-ranking officers would be promoted to their position from slavery.
According to the episode "The Disappeared" of the canon animated series Star Wars Resistance, one can voluntarily enlist as a First Order trooper. While the main characters are interacting with a group of First Order stormtroopers, one of the troopers says:
Remember, each of you can have a future in the First Order, if you wanted.
Keep in mind, these characters are all in their late teens. This clearly shows that so-called older children can voluntarily choose to enlist.