Unfortunately, you probably do not have a viable discrimination case on these facts. We start with the presumption of "at-will" employment under the law, which provides that the employer can refuse to hire (or discharge) an employee for any reason or no reason at all, provided it is not an UNLAWFUL reason.
What constitutes an unlawful reason? Well, that's complicated at times, but generally it is unlawful to discriminate based upon any of the following: race, national origin, ethnicity, color, gender, sexual orientation (in CA anyways), religion, disability, medical condition, genetic profile, Union activity (or other concerted activity), whistleblowing activity, age, or marital status. You'll notice "having an imperfect smile" does not fall into any of these categories, and thus that alone will not sustain a discrimination claim.
You mention having dentures and that this was discussed in the interview. You could perhaps argue that this was a pretext for age discrimination, essentially arguing that they refused to hire you because of your age and the "smile" issue was serving as a proxy for that unlawful motive. Absent more, though, I am not optimistic about the prospects of that argument.
All of the foregoing presupposed that you would proceed under a "disparate treatment" legal theory, which argues that you are in a protected class (like one of the ones mentioned above) and you were treated differently from applicants outside the protected class. There is another discrimination theory called "disparate impact," where a policy that results in de facto discrimination is unlawful even if it does not, on its face, discriminate against the protected class.
As an example of disparate impact, there was a lot of litigation in the 70's and 80's against fire and police departments that proceeded under a disparate impact theory, contending that their pre-application testing requirements were biased against women, African-Americans, etc. Using statistical data, plaintiffs were able to demonstrate in many cases that different components of the testing process resulted in the selection of men or Caucasian applicants in significantly higher numbers than their protected-class counterparts. For example, many fire departments required applicants to drag 150 pound sandbag several hundred yards, to demonstrate that the prospective firefighter could rescue a person from a burning building. This had a demonstrable disparate impact on female candidates, who were less likely to have the upper body strength necessary to complete the task.
I bring this up because it is possible, though you would have to conduct substantial discovery to know for sure, that the "perfect smile" policy has a disparate impact on the elderly, given that they are statistically more likely to have lost teeth, wear dentures, etc.
Once a prima facie case of disparate impact has been demonstrated through the statistics, the burden shifts to the employer to prove that the disputed job requirement is in fact a necessary part of the job. This is known as the "bona fide occupational qualification" (BFOQ) defense. Thus, returning to the fire department example from before, the departments would universally contend that being able to drag 150 pound body several hundred yards was a necessary part of the job and therefore the disparate impact was justifiable.
In your case, I suspect Disney would argue (even assuming you can prove a disparate impact on those over 40 in the first place) that having a good smile is a necessary part of the job and that it forms a fundamental component of the Disney "brand." Las Vegas bars, strip clubs, and the like have all gotten away with gender discrimination for decades on this same BFOQ theory, and I suspect Disney's argument would get some traction.
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The other attorney responding gave a very detailed and good response. Briefly, I will simply say that in general the smile issue or how you look is not an unlawful basis of discrimination. Of course, if you could prove disparate impact (which I doubt), you might have a discrimination claim.