My dog was about 16 (we had her as an adult rescue so not too sure) She slowed down but seemed reasonably happy for a few months and then went downhill quite quickly. One morning her back legs totally went and she looked up at me, just something in the way she looked at me -that was when I knew. Heartbreaking but the right thing to do.
She did manage to get back up and very shakily went outside for a wee, but couldn't walk straight, I called the vets and they couldn't see her for a few hours, she spent those on her mat and was vomiting and couldn't move. Just cleaned her up and talked to her until it was time to go. DH had to carry her to car, she'd lost a lot of weight but was still quite big (border collie)
The vets were amazing, did a quick exam and said things weren't good, they did offer investigation and possible treatment but it didn't feel right to put her through that tbh. Stayed with her the whole time and the vet explained exactly what to expect, was very peaceful and we were offered extra time to sit with her if we wanted.
She'd been very slow, sleeping a lot, no walks etc for quite a few months and we kept wondering if it was time, but it's true when people say that "you just know" That morning there was no wondering, we just knew that we had to do the right thing by her.