I think about that, too. And Smokey isn't even 3 yet. But sometimes when we're hanging out together I think about the fact that he's been by my side since the day I could take him... driving from San Diego to Michigan and back. Moving from SD to NYC. Me n' Smokey. How sad I'll be when he's gone... YEARS from now.
You n' Simba still have many happy years together, too.
On the other hand, I think about when Smokey does have to leave - how great it will be to start a whole new relationship with hopefully many dogs. How I'll be able to finally adopt/rescue dogs and remember that Smokey was the cue-ball head that started it all. :)
(I attached one of my fave puppy pics of him just for fun.)