Went through the same a year ago with our 14 year old English Springer, called Goater. Hardest decision to make. You will know yourself when it needs making as you know her best.
Our oldest girl now, Berry, is almost 15. Sleeps about 18 hours a day, struggles getting up at times (Yumove joint aid tablets have helped her enormously!) and had two strokes in the past, which leaves her balance off at times. Mostly deaf, and beyond a couple of feet can only see shadows apparently. But she still has the desire to want to come to the park, even if I now have to drive her to it, where she plods around happily. She spent the first six years of her life in a shed, on a puppy farm having god only knows how many litters, which has also meant at times she does 'leak'. But we cope easily with using special dog pants, (from Amazon) which hold her pull up Aldi nappy pants in place (Hole cut out for her tail, lol) Hey, we love her, it's the least we can do! :D
She missed Goater so much, but loves playing Granny to the two youngsters we got, Pep and Daisy. (All ES rescues) But we see how tired she is getting, and we know what is going to happen, and the decision will need to make in the likely not too distant future again ....