Most people buy short pyjamas in June, wear them twice during a heatwave, and then wonder why they've gone grey and saggy by September. The trick is they're not a summer novelty, they're the thing you reach for most of the year if you pick the right fabric, and the mistake is treating the shorts like an afterthought when they're doing more work than the top.
They earn their place from about April through to October, for the half hour with tea before anyone else is up, for sitting on the edge of the bed with a fan going, for the nights when a full length pair would just end up round your knees anyway. Cotton jersey is the one that behaves in heat, it breathes, it doesn't cling. What they don't suit is cold feet and cold ankles, obviously, but also anything with a dressing gown that's too warm for the weather, because then you're just hot in a different configuration. Avoid pairing a thin cotton short pyjama set with a thick fleece robe, you'll overheat from the waist up and still have bare legs, which is a strange way to spend an evening. They also don't love being worn under a duvet in a house with proper heating, the elastic waistband starts to feel like punishment by 2am.
How they age depends entirely on what's in them. A cheap cotton blend goes thin at the thigh within a couple of months, especially if you're one of those people who sits cross legged on the sofa, the fabric takes the strain there first. Proper cotton, even a basic one, holds its shape for longer, and a wash at thirty with no tumble dryer keeps the elastic honest, tumble drying is what kills the waistband faster than anything else. Linen shorts look wonderful on the hanger and crease the moment you sit down, which some people love and I find mildly stressful at seven in the morning. Before you buy, check the inseam length isn't so short it rides up when you walk, check the waistband is a proper fold rather than a thin strip of elastic stitched straight on, and check the top doesn't gape at the buttons if there are any, because that's the detail shops never photograph honestly.
What they won't do is double as anything else. They're not daywear, they're not something you answer the door in without a second thought, and they're no good at all once the clocks go back and the house gets properly cold. For six warm months, though, they're hard to beat.