Lifespan follows size more than breed fashion
The fifteen-year figure in your question is realistic for many small and medium dogs, while the largest breeds commonly age much faster and may reach their senior years while a small dog is still in mid-life. When you compare breeds, use the lifespan typical for that size of dog rather than the oldest example anyone quotes, and think in life stages: a brief, intense puppyhood, a long adult plateau, and a senior phase whose length and demands vary hugely. A breed that fits your life at two years old but whose elderly version needs lifting, ramps, and constant veterinary support asks a very different question of you in a decade's time.
Age at adoption changes the shape of the commitment
A puppy is the maximum commitment: the full lifespan ahead, with his adult size, temperament, and health still partly unwritten. An adult dog shortens the unknown list considerably, because what you meet is largely what you get, and the remaining years are easier to picture honestly. An older dog shortens the commitment further, which suits some life stages better, but concentrates veterinary care and eventual loss into a nearer window. None of these is the right answer in general; the useful exercise is placing each option against your own next fifteen years, including moves, career changes, children arriving or leaving, and your own ageing and health.
Plan for the senior dog while choosing the young one
The part of lifespan planning most people skip is the far end. A dog who will one day have stiff joints, fading sight, more frequent vet visits, and a need for ground-floor living should be chosen by someone whose housing and budget can bend that way. Ask breeders and rescues what ageing typically looks like for the breed or type you are considering: which conditions are common in their later years, how large the dog will be to manage if his mobility fails, and what daily care old age usually involves. Choosing with those answers in view is not pessimism; it is how a dog stays well kept for his whole life instead of becoming a problem his owner never foresaw.
Your circumstances will change inside his lifetime
Fifteen years is long enough for most lives to transform, so build flexibility into the plan rather than assuming today repeats. Think about who would care for him if you were ill, whether your work could change in ways that alter his routine, and whether you would move somewhere his size or breed complicates. Dogs cope with change far better than people fear, provided their core relationships and care stay consistent, so the real planning question is not whether life will change but whether you have people, savings, and habits that can absorb change without his care being the thing that slips.
One record that lasts as long as he does
Across a long life a dog accumulates vaccinations, test results, operations, insurance details, and notes from several different clinics, and that paper trail becomes hardest to reconstruct exactly when an elderly dog needs it most. KinsLe's Pet Passport holds his records together in one portable packet from the start, ready for vet visits, travel, boarding, and handoffs at every stage, so the puppy or adult you choose today is still fully documented when he is the grey-muzzled senior you planned for.