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Because you're essentially saying: All other things equal, I'll choose the candidate based on their age.

No, GP is essentially saying: It's clear one accomplishes more in a smaller amount of time. That's far different from all else being equal. Granted, using age has less value than using time-in-field, but they are often proportional. Also, granted, you could argue that amount accomplished is a poor metric to use, especially when joined with speed.

That's exactly what age discrimination is.

What can you infer about learning speed based on age that you can't infer based on years of experience?