lens - Do lenses lose sharpness when they age?

I was reading this answer where it mentions, "..it's a lens that can lose sharpness as it ages.." and was wondering if this is true and if yes, true for all lenses. Reasons as to why this happens and ways to prevent it if possible from a consumer's perspective would be great to know.

Please note, I am not talking about general wear and tear, or dust inside the lenses, nor am I talking about fungus in the lens.

Answer

Given that you have explicitly disqualified fungus and dust inside the lens, then the answer is no. A lens will not "naturally" lose sharpness with age. Glass is glass. It is a fixed medium, and assuming a 100 year old lens is in good condition without any extraneous wear and tear like fungus, dust, or a strong enough jolt to misalign one of the internal elements or element groups...a lens will not lose sharpness with age. A 100 year old lens should perform just as well at 100 years old as it did on day one.

One thing that probably should be pointed out is that lens design has improved over the years. While certain classic designs are often held in lofty positions as they were revolutionary in their time, they may not be the best designs still today. New materials sciences, (frequently pioneered by Canon...such as fluorite elements or diffractive optics), even other general innovations like image stabilization, multicoating and nanocoating, aspheric and apochromatic lens elements, etc. have all lead to progressively improving sharpness over the decades.

With improved lens technology comes improved image quality. Even if a 100 year old lens is in pristine condition, it likely lacks the purity of glass that a modern lens has, probably lacks multicoating and probably even a single coating, and each lens element may not be as precisely aligned as we can do today. Two similar lens designs, one from a century ago and one from today, would likely not be directly comparable. The modern lens design should be producing much higher image quality than the century-old lens.

That does not mean the old lens as "lost sharpness", however...it just means it doesn't compare to a modern lens design that's benefited from all the modern advancements in materials and optical sciences.