Well I cannot say for dogs, but my rabbit is being litter trained now, and she is three years old. She is doing well, but it does take time.
The House Rabbit Society said that it is easier to teach an older rabbit to be litter trained than a younger one.
I'm assuming it would be the same for dogs?

Puppies are easily distracted, IMHO, because they are young and playful. I think it would be easier to train an older dog because they aren't as playful and energetic as puppies. (IMO)