I definitely would not assume that she is dying of "old age", as they can live to 15 years or older, and often do. My first dragon lived to be 13, and he simply died in his sleep, and was completely healthy. So you shouldn't look at an illness as "He's old and probably dying", but rather look at like you would if he was only a year old, like "He's sick, let's figure out what's going on and try to help him".
Have you checked the temps inside his tank, specifically his Basking Spot Surface Temperature and his Hot Side Ambient (air) Temperature, since he's staying out of the Hot Side of the tank and trying to cool himself off in his water dish? (Obviously you must be using either a Temperature Gun or a Digital Probe Thermometer to measure his Basking Spot Surface Temperature, not a stick-on thermometer) And have you tried giving him
hydration several times throughout the day by dripping water or Pedialyte on his snout for him to lick off? Is he eating normally? Has his UVB tube been replaced on-time? There is usually a reason that they suddenly do not want to be inside the Hot Side of the tank, and it's typically related to the temperatures within the Hot Side of the tank, or the lighting over his basking spot..