My current ons are quite young, but we have had much older ones previously and they were fine.
So, have your actually had kidney damage from layers?
BYT experts seem against layers feed in general! I would agree with them, but no one gives me any evidence to do so.
Sulky, one study of many. Poultry Science - Note that this was NOT testing whether excess Calcium in non-laying birds was damaging to the kidneys - as that is accepted science - rather it sought to determine if acidification of the feed (similar to the ACV recommends we see on this forum with frequency) would remedy the damage. tl;dr: "in part".
Here's the Abstract:
Experiments were designed to evaluate the effect of dietary acidification on the development of kidney lesions induced by excess dietary calcium (Ca) and Gray strain infectious bronchitis virus (IBV). Specific pathogen-free (SPF) chicks and SPF chicks inoculated with Gray strain IBV were fed one of three diets: a commercial pullet grower ration (1% Ca); a commercial layer ration (3.25% Ca); or layer ration plus .5% ammonium chloride (acidified layer ration). Gray strain IBV significantly reduced total kidney weights in males, reduced total kidney weight as a percentage of body weight in males, increased the number of gross kidney lesions, and decreased the number of filtering nephrons when compared with uninoculated birds when both groups were fed the grower ration. The layer ration induced a 60% incidence of kidney lesions, caused a significant increase in kidney weight asymmetry ratios, and caused a 25% reduction in the number of filtering nephrons. Acidifying the layer ration significantly reduced the incidence of gross kidney lesions and reduced kidney weight asymmetry ratios, but did not prevent Ca-induced reductions in filtering nephrons.
Evidence is trivially located, and free for your reading. These are not extraordinary claims requiring incredible proofs, rather they are well established facts of known pathology and disease progression, against which you would set your anecdotes.
Those of us using All Flock/Flock Raiser with free choice oyster shell have no need to repeat this experiments to establish for ourselves their veracity in order to satisfy your curiosity. I hope this sates your appetite for the issue, as well.