The moment the evenings start closing in and you need something on under a coat before the heating goes on properly, the jumper comes back into its own. This is the first week of October stretch, when a cardigan feels too thin and a coat alone feels like overkill, and a good jumper sits in between and does the job on its own some days.
Keep it away from anything else with volume or texture competing for attention, so no bulky scarves wound twice round the neck, no big statement necklaces that get swallowed by the knit, and be careful pairing a chunky one with wide trousers or you end up looking like you're wearing a duvet from the waist up. A fitted bottom half balances it out far better. As for how it ages, a decent jumper softens with wear rather than losing its shape, though the cuffs and hem are usually the first to tell you how it has been treated. Wash it cold, on the wool or delicates cycle, and dry it flat rather than on a hanger, because hanging stretches the shoulders and you will never quite get that back. Pilling happens to almost everything eventually, but how quickly tells you a lot, cheap acrylic blends go bobbly within a month of regular wear, while wool and good cotton blends take much longer and respond well to a fabric comb when they do.
The difference between one that lasts and one that doesn't usually comes down to what you can't always tell from a photo. Pick it up and check the knit is dense rather than see through when you hold it to the light, look at the ribbing on the cuffs and hem for tightness, since loose ribbing loses its grip within weeks, and check any seams at the shoulder sit flat rather than bulky. A good label will tell you the wool or cotton content honestly rather than burying it under a wall of polyester. For fit, go by the shoulder seam first, it should sit at the edge of your shoulder, not halfway down your arm, and then check the sleeve length settles just past the wrist bone. Too generous through the body and it reads as shapeless rather than relaxed, too tight and it will ride up every time you reach for something on a high shelf. If in doubt, size up for a looser style and down for anything ribbed, since rib knits give more than they look like they will.
Thrown over a plain white tee and your oldest jeans, it makes an outfit you already trust look like you thought about it that morning, even on the days you really didn't.