Especially in the elder years a goal is to have a very close relationship with your pet's care provider and plan ahead. Loss of an elder pet is a natural part of the circle of life. And sometimes the end isn't at all what was expected. Perhaps it's based on mobility challenges, or cognitive challenges. It can be tough to reason through euthanasia with a pet that is fully cognitive but no longer fully mobile and vice versa. A pet parent will do best to strike a balance between not saying goodbye too soon, and not saying goodbye too late, and make a plan of action ahead. Regardless of timing, a good pet parent will often question their timing. This is completely normal.  When embarking on that important day of saying goodbye it's important to set aside our own emotions, just for now, just for today, to deal with another day. To allow ourselves to stay focused on what our pet is experiencing, and to celebrate their last day, instead of making it one where they are concerned for your stress or your emotions. Your pet knows when you are happy or miserable. He or she worries for you, more than they worry for themself. Be happy in their presence, focus on celebrating their life and staying upbeat, for them, up to their final minute. There will be time to reflect later, when your pet is no longer suffering needing your undivided attention.

Please do not take your animal to the shelter to be euthanized. If you are not able to be present with your pet, a care provider will stand in for you. Every veterinary clinic will prioritize an animal that is suffering poor quality of life.  If cost is an obstacle there are low cost options available. After hours service can be obtained through area emergency pet hospitals. There are at home service options for those who would like to go that route and have the means to do so - these are scheduled services rather than emergency services=be sure to check them out and reach out to them to prepare a few days ahead, even when the exact date is not known.
More info here-  Compassionate Pet Vet and the network of DVMs of Lap Of Love 
​A quality of life scale from Lap of Love can be found here & daily assessment from the same organization.