On the contrary, opinions like religion and politic affiliation are the exact opposite (and that you find those conflated in things like CoC troubles me).
No, age and veteran status change over time. It's true that in the direction they are legally protected, they are mostly one-way gates into protected status (you can become a veteran or a person over 40, but you don't normally exit either status; it's theoretically possible with veterans status, but extremely rare.)
Physical and mental disability, citizenship status, are also protected and non-static.
In California law, marital status, medical conditions, HIV/AIDS status, and status as a domestic violence or stalking victim are also non-static and protected.
Of course your age change, and a veteran once was not ; but try to get younger if you want? nope. Un-AIDS yourself by the force of will? nope
If you have no control over it, that is very static, even if next year you'll be one year older.
But one can decide not to be a nazi anymore, or a cult believer, at anytime. That's where the fundamental difference resides. And these different things should be treated differently.
A real problem for me is to have mere opinions protected from critics on the ground of fighting discrimination. It should always be possible to say to someone that what they believe is wrong or simply absurd.
In any case, the idea of a "protected class" is not "based on being something you can't change about yourself".
That you may want it to be something else is irrelevant. What you believe is wrong and absurd (using your terms), based on the decades of established law and court cases on the topic.
You wrote: "mere opinions protected from critics on the ground of fighting discrimination. It should always be possible to say to someone that what they believe is wrong or simply absurd"
It is (almost) always possible. The question is, should that expression of an opinion be free from negative consequences? The answer is, for the most part, "no".
Furthermore, having "mere opinions" is one thing, while expressing those opinions are another. You might be of the opinion that women are incompetent and should not be hired, and if a women is hired you might express that general opinion to her.
However, as https://www.eeoc.gov/laws/types/sex.cfm points out, in the workplace "it is illegal to harass a woman by making offensive comments about women in general."
(Note: "Unless the conduct is quite severe, a single incident or isolated incidents of offensive sexual conduct or remarks generally do not create an abusive environment. As the Court noted in Vinson, "mere utterance of an ethnic or racial epithet which engenders offensive feelings in an employee would not affect the conditions of employment to a sufficiently significant degree to violate Title VII." ... A "hostile environment" claim generally requires a showing of a pattern of offensive conduct". https://www.eeoc.gov/policy/docs/currentissues.html )
There's plenty of examples in the EEOC guidelines to help understand when something goes beyond "mere opinions" and into the realm where the courts may decide that a given workplace environment is "hostile or offensive".
You can't change your date of birth or fact that you were in the armed forces. That is static. Religion is constitutionally protected.
