Trainers used to mean one pair for the gym and one slightly cleaner pair for everything else. That's gone now. They sit under dresses, under tailoring, under things they'd have looked wrong with a decade ago, and I think that shift has been good for everyone's feet and most people's style.
The difference between a good pair and a disappointing one shows up in places you don't notice until month three. Look at where the upper meets the sole, a glued seam will start lifting at the toe before you've had much wear out of it, while a stitched or properly bonded join stays put. Run a thumb over the eyelets and the back heel tab, cheap hardware goes dull or rusts if you're caught in rain, decent metal doesn't. Canvas should feel tightly woven rather than papery, leather should have a bit of give rather than that stiff, shiny coating that never softens. Soles matter too, a chunky sole with proper tread will keep its shape, whereas the ones that look dramatic but are made from soft, cheap foam start sagging at the sides within a season and suddenly you're walking on the outer edge of your own shoe.
How they age depends entirely on what you do before you've even worn them, which sounds dull but it's true. Leather and canvas ones benefit from a protector spray on day one, before any scuffs happen, not after. White ones show every mark from day one regardless, so a magic eraser and patience become part of your life, that's just the deal you've signed up to. Running ones lose their cushioning quietly, no drama, just one day your knees notice, so they're not a forever shoe even if they look fine. Platform and chunky styles take the most visible wear at the back corner of the sole, where you drag your foot getting out of the car, so check that spot before buying if you can. For fit, go by how they sit at the heel first, not the toe, because a gap there means blisters by lunchtime no matter how much room you've got up front. Thick socks for running ones, proper socks for trying on, and remember most trainers run a touch big rather than small, so don't automatically size up out of habit.
They work hardest from fairly early spring through to late autumn, then boots take over, though canvas ones rarely cope well with proper rain regardless of the month.
Stuff them with paper rather than leaving them to dry flat, it keeps the shape while they dry out properly.