1. Dementia - aka canine cognitive disorder can happens as early as 9 years or as old as 14 years or more.
Google for Canine Cognitive Dysfunction Checklist, see how many ticks your dog has. Cognitive Dysfunction Syndrome, also referred to as “sundowner syndrome because some dogs will start pacing up and down, or bark at night. Another common symptom is getting stuck in corner.
2. Arthritis can starts from 7 years with sore muscle or as late as >10 years. Acupuncture, hydrotherapy, massage and warm compress are common treatment to alleviate muscle soreness. Like all humans, the spine will get weak as we/dogs age, which results in weakness of hind leg support. Gradually, they will put more pressure on the front paws. And slowly muscle degenerate all the way to neck area. Food is very important during senior years, you need to provide a lean protein diet due to muscle loss. Also, invest in an elevated food and water bowl to reduce straining.
3. Kidney failure does not happen in all dogs, likewise, not all dogs will get cancer. This can happen at any age. Senior dogs are recommend to take full ultrasound test ($350+++) every 6 months/year. Blood test does not give you much information on the health.
Aging in dog is similar to humans.