My beagle mix Lou turned twelve last spring. I noticed the changes before his annual bloodwork confirmed anything — slower to get up from his bed in the morning, less enthusiasm on the stairs, occasionally stiff after a long walk. His arthritis is now managed with a multimodal protocol and he's doing well. But if he were a patient rather than my dog, his owner might have written off the slowness as "just getting old" and not brought it up at an appointment.
That distinction — between changes that are genuinely inevitable and changes that are manageable if caught — is the whole point of this article. Many of what we call "normal aging" changes are actually treatable conditions that, left unaddressed, reduce quality of life significantly.
Musculoskeletal System
Osteoarthritis is the most prevalent age-related condition in both dogs and cats, and it's also the most systematically underdiagnosed. Studies suggest the majority of cats over twelve years old have radiographic evidence of arthritis — yet many owners are unaware their cat is in pain.
Signs in dogs include stiffness after rest, reluctance to climb stairs or jump into vehicles, reduced tolerance for walks, and visible favoring of a limb. In cats, arthritis often presents more subtly: not jumping to high spots anymore, reduced grooming of hard-to-reach areas, changes in litter box use (difficulty stepping over a high-sided box), or general quieter behavior.
Muscle mass loss (sarcopenia) accompanies aging in both species. Dogs and cats lose muscle mass progressively, which contributes to weakness, reduced stamina, and difficulty with temperature regulation. Maintaining appropriate protein intake and regular gentle exercise helps slow this process.
Bone density changes occur, though pathological fractures are uncommon in companion animals unless significant underlying disease is present.
Organ Function
Kidneys
Chronic kidney disease (CKD) is one of the most common age-related conditions in both senior dogs and cats, with cats disproportionately affected. Some estimates suggest CKD affects the majority of cats over fifteen years old.
The kidneys lose nephrons (functional units) with age, and the remaining ones compensate until they can no longer maintain normal function. The critical window for early intervention comes when kidney function has declined but clinical signs haven't yet appeared — which is why routine blood and urine monitoring matters so much.
Early signs at home include increased water consumption and urination, then later poor appetite, weight loss, vomiting, and lethargy.
Liver
Liver enzyme elevations are common in older dogs and reflect a range of changes from benign nodular hyperplasia to early hepatic disease. Many older dogs have mildly elevated liver values that remain stable and require monitoring rather than treatment. Others develop progressive liver disease that requires management.
Heart
Mitral valve disease (MVD) is the most common cardiac condition in older small-to-medium-breed dogs. Affected dogs often live for years with managed heart disease, but it requires monitoring with physical exams, chest X-rays, and cardiac ultrasound to guide medication adjustments as the disease progresses.
Cardiomyopathy — particularly hypertrophic cardiomyopathy (HCM) — is the predominant cardiac disease in cats. It can be present without obvious symptoms for years. Some cats are discovered to have heart disease for the first time when they develop a sudden breathing crisis or hind limb paralysis from a thromboembolism.
Endocrine System
Hypothyroidism in dogs — underactive thyroid — becomes more prevalent with age in certain breeds. Signs include weight gain, lethargy, cold intolerance, and coat changes.
Hyperthyroidism in cats — overactive thyroid — is extremely common in cats over ten years old, likely affecting around 10% or more of older cats depending on the population. Despite its prevalence, it's often not suspected until clinical signs (weight loss despite good appetite, hyperactivity, vomiting, or elevated heart rate) prompt investigation.
Diabetes mellitus increases in incidence with age in both species. Obesity in middle age is a significant risk factor.
Sensory Changes
Vision: Nuclear sclerosis — a bluish haziness of the lens — is almost universal in dogs over eight years old. It looks similar to cataracts but has minimal effect on vision. True cataracts, which do impair vision, also increase with age. Glaucoma, retinal degeneration, and other eye conditions are more common in seniors.
Hearing: Progressive hearing loss is common in older dogs and is often not recognized because dogs compensate remarkably well with their other senses. Many owners discover their dog is significantly hearing-impaired only when the dog stops responding to their name or seems startled when approached from behind.
Smell: Olfactory function declines with age, particularly in dogs. Since smell drives appetite, diminished smell can contribute to reduced interest in food.
Skin and Coat
The coat of an older dog or cat typically becomes coarser, less shiny, and may show areas of thinning. Skin becomes less elastic and more prone to benign growths (warts, sebaceous cysts, skin tags). Regular examination of the skin during grooming helps identify new lumps early — important because while most are benign, some are not.
Grooming behavior changes are also worth noting. Cats that suddenly stop grooming as thoroughly as before may be dealing with arthritis pain (reaching to groom is difficult), dental pain, or systemic illness. A cat that begins over-grooming may be stressed or dealing with a skin condition.
Cognitive and Neurological Changes
Cognitive dysfunction syndrome (CDS) — an age-associated neurological decline — is one of the more distressing changes for families. Signs include:
- Disorientation — staring at walls, getting stuck in corners, seeming confused in familiar spaces
- Altered interaction — less engaged with family, or conversely, becoming more clingy
- Sleep-wake cycle disruption — sleeping more during the day, restless or vocalizing at night
- House soiling in previously reliable pets
- Forgetting learned behaviors
It affects a significant percentage of dogs and cats over twelve to thirteen years old. While there's no cure, early management with environmental enrichment, dietary interventions, and in some cases medication can slow progression. The geriatric screening tests that detect these changes early include neurological assessment that can help characterize cognitive changes.
Digestive System
Gut motility and digestive efficiency change with age. Constipation becomes more common, particularly in older cats. Appetite regulation shifts, and some senior pets become more finicky or less food-motivated than they were in youth.
Dental disease — technically a mouth health issue but profoundly affecting the entire GI system — is extremely prevalent in seniors and often a source of chronic pain that suppresses appetite and degrades quality of life. Regular professional dental care is one of the highest-impact interventions available.
Immune Function
Immune function declines with age (immunosenescence), making older pets somewhat more susceptible to infections and potentially affecting vaccine response. This is another argument for maintaining core vaccine schedules as recommended by your veterinarian rather than discontinuing them in old age.
Putting It Together
For families with senior pets, the comprehensive guide to senior pet health provides the full framework for monitoring and management. And for understanding exactly when a dog is considered senior by breed size — because it varies more than most owners realize — that piece covers the details by breed category.
The thread that connects all of these changes is this: age-related changes are not inevitable sources of suffering. With proactive monitoring and appropriate intervention, the vast majority of senior pets can live comfortably and meaningfully well into their final years.