Foaming at the mouth and convulsions are not symptoms of dying of old age. It sounds like your dog is having a grand mal seizure which should be treated urgently with medication administered by a vet. Keep your dog warm. I recommend you take her to a vet immediately. Wrap her in a warm blanket and keep her warm on the way to the vet.
Grand mal seizures can be due to many causes e.g. eating something toxic, a head injury, an electrolyte imbalance due to various diseases, excessively high temperature above 106 degrees F, dehydration, and other causes too numerous to state here.
When a dog has a grand mal seizure the dog's jaw muscles will spasm and the dog will appear to be chattering its teeth and then the dog will begin to salivate. Breathing will become forced and if the jaw is set in a closed position the forced breathing will stimulate the saliva to foam up and the dog will appear to be "foaming at the mouth." http://www.thepetcenter.com/gen/epilepsy.html
When a dog dies at home the dog dies slowly and peacefully. A dog that is dying from natural causes (old age) does not foam at the mouth and convulse.
If your dog stops breathing then perform CPR http://www.dog-first-aid-101.com/dog-cpr.html
If you have any questions please ask me.
Has she eaten anything recently? When was her last meal? What did she eat? Has she had a normal bowel movement every day? Has she been urinating normally (is her urine clear and not orange)? Has she ever been diagnosed with any chronic problems e.g. diabetes, kidney disease, or epilepsy?
When she becomes responsive then offer her a bowl of warm (baby bottle temperature) sugar water. Mix 4 to 6 tablespoonfuls of sugar into 1 or 2 cups of water preferably while the water is boiling so the sugar is uniformly mixed into the water and doesn't settle on the bottom of the bowl. Place some of the sugar water mixture on her lips and if you can then place some inside her mouth on her tongue. Rub her chest gently and massage her body. This should help arouse her.
Also, it's very important to keep her very warm. Warmth will help her circulation and help her to become more responsive.
The ground is cold. Place her on a soft surface and keep her warm by snugly wrapping her in an electric blanket set to medium if you have one.
I recommend you wrap her up very warmly and then take her to an emergency vet. Telephone the vet beforehand so the veterinary team will be prepared to treat her immediately when you get her there. She should be given IV fluids with dextrose to help arouse her.
Please keep me informed.
I am so very sorry we couldn't save her. You did all that was humanly possible to save her. She is at peace.
I am sorry.