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There are several members here that have had hens as much as 3 years old suddenly grow spurs after their rooster died, and though I don't have any links handy research has been done on the subject. Hens who make this trans-gender switch have also been known to start crowing, while still laying eggs everyday.
There is no doubt that there are cases where genetics are the only factor to influence spur growth in hens, but genetics are not the only factor in every case.