The pants—and the person wearing them—are about 3,000 years old, give or take a few decades. No archaeological evidence of older pants have been found. Even Ötzi the iceman was only wearing a combination of loincloth and individual leggings, which is just so terribly 5,300 years ago.
These pants, which were recovered from a tomb in China, are about 400 years older than the previous record holder for "oldest pants," which were found on Cherchen Man, who was buried in the same area.
The tailoring process did not involve cutting the cloth: instead the parts were shaped on the loom, and they were shaped in the correct size to fit a specific person. The yarns of the three fabrics and threads for final sewing match in color and quality, which implies that the weaver and the tailor was the same person or that both cooperated in a highly coordinated way. The design of the trousers from Yanghai with straight-fitting legs and a wide crotch-piece seems to be a predecessor of modern riding trousers.
The owner of the pants was likely a warrior in his mid-40s and was buried with other horse-related implements, including a bit, whip, bridle and a horse tail, in addition to weapons. Horses were obviously important to the culture that buried this individual. Scientists believe that horses were first domesticated somewhere in Central Asia between 4,000 and 3,500 years ago, and it is likely that trousers were invented soon after the first human figured out that horses were really good at carrying people on their backs.
It makes sense that people would develop a way to ride horses comfortably soon after horses were domesticated. Riding a horse in a skirt before a proper sidesaddle was invented? Ouch.
If you want to see modern recreations of the kinds of clothes worn in China 3,000 years ago, like the pants, you only have to wait until 2017, when researchers from Germany and China plan on organizing an international fashion show showcasing what people on the Silk Road wore three millennia ago.