I came into some discussion with my friend, and my friend said that age may affect the color produced by the lens, i.e., a newer lens produce better color than an old lens (of the same model). Is this true? And why is it so? What component in the lens that is subject to wear and tear in a way that affects color?
In general, lenses do not degrade in this way. However, some old lenses made up until the 1970s contain elements made of thorium oxide — a radioactive element which yellows as it ages. These were used because they have a different refractive index, with low dispersion — so, generally, better-quality lenses, just as fluorite crystal or other exotic elements are used today. It's also possible that adhesives (used to bond lens elements to each other) or coatings used in older lenses may yellow as they age. In both of these cases, if there's enough effect to make a difference, you should see it when just looking through the lens.
The other difference may be in newer lens designs vs. older ones. This shouldn't affect the case your friend is talking about (same model over time), but may be the source of the confusion. Design of coatings changes, as does the types of glass used. And this can be very strong between manufacturers — for example, while I haven't found this anywhere official, many people believe Sigma intentionally introduces a slight color warming effect in their lenses.
Nonsense. Lens glass does not generally discolor over time, and the coating is very thin. Even if some discoloration occurred, you could easily see this simply by looking thru the lens. I have some old lenses, some made before WWII, and none of them are tinted to the point where I can notice this by looking thru them.
Even if some discoloration occurred for some reason, it would be slight enough to be correctable. The difference between lenses would be significantly less than the difference in the color of ambient light. Your existing methods of dealing with ambient light color variations will cover any additional slight shift due to lens discoloration. Take a white or gray reference shot as usual, and all should be well.