Religion is a legally protected class; everyone is free to change their religion. Pregnancy is also a legally protected class, despite being one that half the population can enter and leave. Also, short term disabilities.
I don't find this argument compelling, unless you're also arguing that employers should be able to pay people less for their religion or because they get pregnant (or choose not to get pregnant) or because they contracted a disease or injury.
That's a different use of class, but an important one too. We want people's choices around families and religion to be free from outside coercion, so we have special protections. Short term disabilities are not a choice but also non-universal, and without the protection they might worsen into long-term ones.
But being a teenager is a not a choice, and not something that happens to some people but not others. Again, there's many reasons we don't want to promote shit jobs for adolescents because of some nonsense about character building, but I think that's a rational distinct from the others.
Seems like you're reaaaally splitting hairs here. Young age is okay to discriminate against, but not old age (because you don't leave old age without dying). Anything people can enter and leave by choice is not okay to discriminate against if it's -important-, like family or religion.
What about menstruation? That's pretty universal to women (not entirely especially with non-cisgendered), not really a choice (at least, not without encroaching on the same protections you'd want with pregnancy), and all those who do menstruate enter and leave it (both monthly, and at menopause)...what's your objection to discriminating against menstruating women?
I don't find this argument compelling, unless you're also arguing that employers should be able to pay people less for their religion or because they get pregnant (or choose not to get pregnant) or because they contracted a disease or injury.