Welcome Candice!
Well, I have to say you are certainly blessed to have a Sheltie reach the age of 15!
In all my years owning Shelties the age of 14 is when I would loose them.
In regards to your question, yes, I have had a few that did have potty issues. My first recommendation would be to see your vet to rule out any medical issues.
After ruling out the medical issues, I have had to reinforce the potty training using a crate. Not sure if your pups were ever crate trained. If they were not I would not try this method as it may do more harm than good.
My Baron is 7 and still sometimes poops in the house overnight. He came from a kennel and was never really potty trained properly.
If urination is the issue, possibly a doggie diaper might work? I had to use it on my other male Sheltie as he got older. Very infrequently but when he had a problem I would use it.
Good luck and let us know your progress...