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.
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.
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?
By definition, age is not a class, because by definition, /everyone moves out of that class/. Your entire proposition is faulty from the foundation!
A better path would be for employers to have internal training programs for gaining skills that can help one to climb the career ladder either internally or externally knowing that if a good person leaves that the reputation will attract suitable replacements.
Granted, jobs can be created and the more skilled the labor force the better the jobs, so the number of chairs in the game can be increased, but on the other hand those aren’t the kind of skills you’re going to pick up serving burgers at mcdonalds.
I worked as a mailman between 11-13 y.o. It was a shitty job carrying ads and local newspapers weighting as much as you on a bike through hills in -10C snowy north. A shitty job, but I learned a thing or to; planning, discipline and value of money.
Problem is when you're stuck at a shitty job with no option to change/improve at all. I don't see how that's the case for teenagers at entry level job.
The gap between a juvenile minimum wage and an adult minimum wage should equal the value of the on-the-job training that the juvenile is getting. And there's really no reason to tie that to youth, although demographically it will be dominated by young people; adults might accept a lower minimum wage in exchange for education in the trade.