I have never had a dog before so everything I have had to deal with is new to me. Having said that, Django is 10 1/2 years old and I consider him a senior now. We have noticed subtle changes, he can't RLH as fast or as long as he use to but he still manages to get a circle or two in a couple times a month. He walks around the house a bit slower and always looks for sun spots in the house to lay down in. He doesn't run up and down the stairs as quickly,sometimes there's a bit of hesitation at the bottom of the stair. He also has become a deep sleeper and sleeps more. He still however gets VERY excited every day when we go for his walks but then crashes on the couch when we return.
The other extreme is when he goes from the behavior listed above which is most of the time to a slightly agitated dog that whines constantly for something (I wish he could tell me what he wants) and can't seem to settle down, constantly on edge for a short period of time.
I know his Addisons and all the medication he has been on has probably aged him a bit more but are there behavioral changes that occur as your dog ages?
It sounds as though everything you are describing is part of the aging process and, as you noted, with Django's medical history, it could be happening sooner for him. He has been through so much.
As far as the hesitation at the bottom of the stairs, that may be caused by a vision problem. As they get older, their vision can become impaired and they will have trouble navigating stairs. A couple of years ago, Tyler tried to come up the back stairs onto the deck, miscalculated and knocked out three of his front top teeth. After that my husband built him a ramp and it worked like a charm until he starting running and jumping from it which eventually lead to his torn ACL.
The sleeping more and sleeping deeply is also part of old age. Tyler sleeps most of the day now but also goes for walks, as well as working out 45 minutes a day on his treadmill. He does, though, still RLH, which scares me because I don't want him to wipe out and reinjure his leg. He doesn't do it every day, but too much for me. He also will whine at times and have trouble settling in. This mostly happens in the evening when we settle in for TV. I have read that can be a sign of canine cognitive disorder which could translate to dementia and/or Alzheimer's in humans.
It's hard to watch them get old but as long as they seem happy and are not in pain or suffering in any way, it's a blessing to have them with us for as long as possible. We had Bailey for 15 years and Tyler is about 6 or 7 weeks short of 17. He has been with us since he was 9 weeks old. I was just thinking yesterday that is equivalent to having your children go from birth to a senior in high school. Wow, when you look at it like that, it's amazing!
Lastly, Tyler has always picked the sunny spots on the floor to sleep, but I always attributed that to the fact that he was a Southern dog (VA) who moved to the cold Northeast (MA). Hope this helps you.