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Currently businesses are allowed to discriminate against anyone under 40 on account of age.
Really? Can you cite a source for this, I'm not really buying it...
And most lines of work aren't allowed to hire anyone under 16.
Exemptions can always be built into such an omnibus law, or added via additional laws later if something's overlooked. Laws to allow ageism in limited circumstances (e.g. bars can refuse to hire someone under 21 (and probably are required to)) could be easily added. And we could also add into the "age of majority" rights that one obtains upon their 18th birthday (or whatever) is the right to not be discriminated against for one's age -- meaning that while legally a business could hire a 16-year-old, they're not prohibited from turning down said 16-year-old simply because said 16-year-old is 16 years old.
This isn't even specific to ageism. Sexism is acceptable in some circumstances as well -- such as a strip club hiring only female dancers, or a VAW shelter hiring only female employees, or places like Curves hiring only female workers.
The point is that even accounting for the exemptions of acceptable discrimination, it would be easier to simply pass a blanket "No discrimination" law, than to create the patchwork of inconsistent non-discrimination-only-for-certain-groups-we've-decided-deserve-rights we have today.