Well she does basically sound quite fit and ok for her age, but there are a few things you might want to check out before you go down the behaviour route.
Do you think she might have a little cognitive dysfunction? This can and does happen to older dogs, just as it can to humans. And there will be all sorts of different levels to it, and different effects for each dog that gets it.
Does she, do you think, have any physical problems? Does the urine smell strange at all, or have any blood in it? Is her poop normal?
OK if she seems perfectly healthy physically, then did she ever have any housetraining problems before? Or has she always been fine, and this change is recent?
Of course it could just simply be that she hates the weather, is too cold to even consider going outside, and just decided it's more comfortable to do her business inside until it has become a bit of a habit.(which is still unusual for a housetrained dog, and could show some cognitive decline) However once the smells keep being reinforced the habit becomes ingrained.
Is there a place you could let her do her business....say in a covered porch or "mud-room" perhaps in a large litter tray? And try to encourage her to use that by the usual system of praise and reward. If she is going deaf and can't see very well, it will be up to you to make sure she does start using it, to get the smells onto it. (I'm not talking bad smells here, but the scents a dog would pick up, so even if the litter was freshened, she would still get the scent) Praise and reward could be physical affection (kiss her head, rub her ears, stroke her....etc) plus a little food treat.
You'll certainly need to rule out a physical problem first. A dog is considered a senior by about 7 years of age. That's when we, as responsible owners, should start doing anual lab work to look at basic organ function of things like liver, pancreas and kidneys. Kidney failure can cause all kinds of odd problems with behavior and elimination because the kidneys can't filter out toxins in the blood. Same thing goes for the liver. Your dog simply may not have the ability to control elimination at this stage of life.
It's time for a thorough workup with your vet. If everything else is fine, then look into behaviorial issues. In the meantime, have you tried some of the puppy training pads? They're scented to attract the dog to eliminate on them. Always work with the dog's nose first since that's the strongest sense they have. You could even start by placing a paper towel soaked in urine on the training pad to help her get the idea.