He had a accident: How do you know when it's time?

[buttercup132]

Thunder isgetting really old now and it just seems like he is in pain. But he has your typical old dog things like artheritst,hip displaysia,thyroid, he has trouble getting up from laying down, the stairs are a problem and he is really skinny now (like Rocky the Rotty). He is also starting to loose his hearing.
How do you know when it's time to put them to sleep when they are so old?
I think he's 13 now.


UPDATE: He came in my room tonight and layed down and just started to pee. He didn't ask to be let out or cry or anything, and actually he pawed at my door so he could be let in.