LOL To not doing windows! LOL

I don't know the answer to that Cheryl but I can link you to the percentage of dogs who don't make the cut in service dog organizations such as Paws With A Cause. Scroll toward the bottom for their averages. They don't take dogs over two years of age.

Paws With A Cause

I think it is a matter of what exactly does the dog need to do to provide assistance to their owner? If it's a companion dog I believe one can be trained in basic obedience. If it's a dog such as our Bergie, she does well simply picking things up for Jim and opening doors. Those type of assistance dogs I think can be trained at any age. If one needed a full assist dog who can call 911 and such I would suggest starting from a puppy or less than one year old to get the best success.