For seniors who shiver, or who don't move much any more, who don't want to go outside for their walks anymore for the cold, or who just like it warm. Price range varies greatly.

Criteria: The coat should be easy to put on and take off. Thigh muscles and chest covered for warmth and prevention of injury from cold muscles. So the popular coats in the style of horse blankets aren't on this list here. The fabric should be comfortable to wear, not stiff, and be at least wind and snow repellent. Here are my favorites:

These Swedish coats are individually adjustable, keep the major muscle groups warm, and are made of top performance fleece. Great looking, too. Chip has the Rogle, and it still looks new going into its fourth winter of daily winter wear on long walks in the woods. It was pricey but well worth it: (Polartec® Thermal Pro™) (Polartec® Wind Pro™) (Polartec® Power Shield™)
I ordered directly from Sweden (25 % VAT is deducted for orders from outside the EU, shipping is 19 Euro).

PetEdge has a snowsuit with zippered legs and hood, warm and easy to put on, not waterproof:
Nannie Millie recommended!!! I wish it would fit the Chipster but isn't made for square framed dogs, and it's not adjustable. Nannie Millie had the blue one, you can see her wearing it in the Nannie Millie book!

Clean Run, the agility site, has two by D-fa that fit the bill:

SmartPak has one that covers chest, belly, and at least part of the thigh:

RuffWear has two that cover the chest but not the thigh muscles:

Not easy to put on, so not good for seniors, but otherwise great are these one-piece snowsuits made of wind and snow blocking high tech fleece (like the Obtrack Skrylle)

Fido Fleece are step-ins with a velcro closure on the back. They cover the chest but not the thigh, and tend to be short in the back for a GSD frame, but worth a look if you prefer to let the dog wear the coat over a mobility aid harness:
The fleece is warm but not wind proof.

PetEdge copied the Fido Fleece design:

Please add others that you found and that worked for your dogs!

I never thought I'd put clothes on a dog, but had to take a crash course in all breed rescue with the many short haired dogs. It came in handy in senior GSD care here in the NE winters, brrrh. And for Chip who was found with his wire broken coat permanently damaged. I need to put a warm sweater on him before he even goes out to the yard when it's as cold as it is now!
Chip vom Dog Pound, border terrior, Director of Rodent Control, CGC
Elly von Rescue, GSD, CGC, HI(C)
Captain Jack, Mama Blue, Little Orca, Bübchen, Tux, Cherub (pocket lions)