It is a fact that nacre can wear away in a tube shape, particularly if the wearer perspires a lot. It wears off the part of the pearl that has the most contact with the skin. So the roundest, or high point of the pearl comes off first and spreads as more of the pearl is exposed to the skin. The less round they are, the faster it can happen.
Some people have drier skin with a good Ph that simply can't hurt a pearl no matter how many years are involved and some people are caustic and can destroy a necklace in a couple of years.
This guy was right that it is mostly seen in old necklaces, but it certainly isn't universal.