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It's almost as if reality doesn't conform to easily applied one-size-fits-all rules for every situation.
To elaborate, your crime example could be supported by evidence that shoplifters rarely reoffend after rehabilitation but pedophiles don't. I doubt this is actually true but taking the view that all arguments must be extended outwards until they fail and then abandoned makes it kind of hard to have any arguments at all.
Yes, I am fully aware that the law is fluid, that was why I specified that age discrimination is legal in most countries in Europe. Germany, for example, has legal protection against age discrimination in the labour market/workplace, and there is every reason to think that similar legislation will be introduced in other countries. Or it might not.
However, your reasoning is off. You try to discuss a constant, discrimination, by comparing a number of completely different things. It will get you nowhere. For example, most countries allow some degree of discrimination against people with a criminal record. The state of New York is taking steps to limit this. Their case against this form of discrimination is made on the back of arguments such as the inalienable rights of the individual, the benefit to wider society etc. And of course one can point to a change in norms, such as a belief that people change.
But if one instead uses your line reasoning by saying "if you deny someone a job on the basis of a past conviction for shop lifting, how is that different from discriminating against the left handed?" then someone else can use the same approach to say "if a convicted shop lifter is allowed to work in a shop, then what's to prevent a convicted pedophile from working in a nursery?".