The bars are the first test she starts failing
A fancy mouse's vertical world is her fitness ledger. An aging fancy mouse with joint discomfort typically gives up climbing before she gives up walking: fewer trips up the cage bars, climbs that end in a slide back down, and hammocks or upper levels left unvisited. Mice use their tails as balance poles and fifth limbs, so watch the tail too — an aging mouse whose tail drags, lashes stiffly, or no longer wraps and adjusts on narrow perches is compensating for a body that hurts to maneuver. These changes are easiest to see during her active evening hours, under dim light, when she believes the world is hers.
Short strides, hunched posture, shorter wheel runs
On flat ground, joint discomfort in an aging fancy mouse shows as short, careful steps and a hunched outline — back arched, head carried low, coat sometimes rough because full grooming stretches have become uncomfortable. Wheel sessions shorten and slow, and she may sit in the wheel without running, a mouse equivalent of standing at the gym door. Because mice are group animals, compare her against her cagemates of similar age: one mouse consistently last out of the nest, avoiding the scramble at feeding time, and choosing the lowest sleeping spot is drawing you a map of what hurts.
Sleep-site and social shifts are mobility data
An aging mouse rearranges her life around sore joints quietly. She may move her sleeping to a ground-floor hide, stop contesting the warm center of the group pile, or nest where food can be reached without a climb. Hoards appear in odd low places. None of this looks dramatic, and all of it is information. In a species with a short lifespan, these rearrangements can unfold over two or three weeks, which is why impressions fail owners: by the time a fancy mouse obviously seems old, the change has usually been running for a meaningful share of her remaining life. Written notes with dates beat memory at this scale.
Slow-downs that are not joints at all
Not every aging mouse who moves less has joint discomfort, and some alternatives are urgent. A fancy mouse who breathes with visible effort, sneezes persistently, or goes off food needs veterinary care promptly — respiratory trouble is common in mice and does not wait. Lumps are also frequent in aging fancy mice and can restrict movement mechanically as they grow, so a slowing mouse deserves a gentle full-body feel, not an assumption. Sudden weakness, a head tilt, or one-sided circling points away from joints entirely. The vet's job at the visit is exactly this sorting, and your dated notes about climbing, wheel use, and breathing make the sorting faster.
One-level comfort while you book the visit
Rehouse her world on the floor of the cage: food, water, nest material, and a low hide all reachable without climbing, with ramps replaced by gentle slopes and gaps between levels closed so a stiff mouse cannot fall. Keep her with her companions if the group is peaceful, since isolation stresses mice and stress masks comfort gains. Handle her over a soft, low surface, and weigh her in grams weekly — in a mouse, losing a few grams is a large event. Bring the vet your climbing and wheel notes rather than a summary. With each week's climbing and wheel observations filed in a KinsLe Pet Passport, her next appointment starts from hard evidence rather than recollection.