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.
To the best of my understanding, not hiring somebody because of age would be age discrimination.
Inferences about learning speed or motivation based on achievements per years don't sound like age discrimination to me.