Earlier quoted context omitted.
Not sure why this getting down-voted. There are issues, like to what extent having teenagers work is useful or harmful to society in the first place, but a universal things everyone enters and leaves isn't a class in the segregated and hereditary sense.
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…
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.