However, it is vital to listen to your what your pet is showing you through their interactions. As animals age, they sleep more but excessive sleeping is a sign their quality is diminishing. The same principle applies to the opposite which is the inability to sleep properly and restlessness/anxiety. Pets can get very confused and senile as they age and this compromises their ability to rest and feel comfortable.
Aging bones and joints suffer from arthritis and this causes inflammation and pain. When you can see your pet is struggling to get up or down from different positions or furniture this can affect their comfort. The basic life functions of eating, drinking, urinating, defecating and sleeping should be a pleasure and not a struggle. When your pet has difficulty with bowel movements or does not want to eat or drink their bodies are telling us they are having a hard time. Appetite and nausea often fluctuate with organ disease but once there has been no appetite for at least 70 % of a normal meal for 48 -72 hours it is often time to make the difficult call.
A lot of animals, especially predator species like dogs and cats do not show weakness until they are really sick. This is pack survival mechanism and protects the others. If your pet vocalizes or flinches when touched or lifted is a sign of pain. Heavy fast breathing or very shallow breathing can be a sign of pain and suffering.
There is a quality of life scale you can use also to help you. You will often know when it is time.