It is my understanding that only cloth/paper made after about WWII will "light up" under a black light. These materials sort of glow under the light, due to artificial whiteners/brighteners. It can be especially apparent in thread used for stitching. So, if something glows under the black light, in can’t be older than when these whiteners/brighteners came into use. But not all new materials will glow. So, if it glows, it can’t be that old. If it doesn’t glow, it may or may not be old.
https://www.sportscollectorsdaily.co...-collectibles/