Pets, on average, age five to eight times faster than humans. By age two, most pets have already reached adulthood. At age four, many are entering middle age. By age seven, many dogs, particularly larger breeds, are entering their senior years. Because pets age so rapidly, major health changes can occur in a short amount of time. The risk of serious conditions increases with age. Annual health checks can help to diagnose, treat or even prevent problems before they become chronic or life-threatening.