Archaeologists recently confirmed that the Tarkhan Dress (which has decayed to just being a shirt at this point) is between 5,100 and 5,500 years old. This dates it to near the beginning of Egyptian society. Of course, it was a shock to find a (mostly) intact garment of such an age, since normally linen is destroyed by the ravages of time fairly quickly. (Fun fact, one of the major forces contributing to clothing falling apart over long spans of time is the friction caused by particles of dust coming into contact with it.)
The Dress was originally found in an Egyptian cemetery in the early 1900’s. Its significance was largely overlooked by the academic community, until 1977, when it was sent to the Victoria and Albert Museum. The long sleeved garment was found inside-out, and had significant creasing, suggesting that it had been actually worn, likely by one of the tomb’s occupants. At the moment, the dress stops at the midriff, but evidence such as similar, slightly younger garments, and images on nearby tombstones indicate that it would originally have been floor-length.
The Dress has long been suspected to date back to the time of Egypt’s First Dynasty, but this was recently confirmed by radiocarbon dating. Radiocarbon dating is a process that we’ve only known about for around a century. It uses the decay of a certain Carbon isotope (isotopes are a type of particle that share chemical properties with each other, but slightly differ in mass) to measure how much time has passed. Carbon-14 is the one carbon isotope that is radioactive — which doesn’t mean it’s dangerous, just that it emits energy — and by finding the amount of it in a substance, we can tell how old it is. Since we have lots of evidence from very old things, like trees, we can use those as reference points to get very accurate dates, to within a few centuries.