When my parents brought Copper home from the shelter, she was an unknown. Had she ever lived indoors? Was she housetrained? Did she guard her food or toys? What would she think of the other dogs? The cats?
Because we didn't know the answers to any of those questions, we assumed she hadn't lived indoors, wasn't housetrained, did guard food and toys, etc.
Instead of letting her run off and explore the indoors on her own, she was leashed inside the house and we accompanied her from room to room as she sniffed and investigated each corner. Instead of waiting for her to go to the bathroom inside, we took her out every hour. Even after she did her business outside, she was still kept on leash inside — there was no telling what she'd get into, what she'd chew, what she'd eat if left to her own devices.
We each took turns taking her outside every hour, and would make note of what she did (pee or poop) outside and when she did it. Thirty minutes after she had her breakfast and dinner, we took her outside, as well. Before long, not only was it easier for us to predict when she'd have to go, but she picked up on the schedule.
After having Copper for two weeks now, my parents have only had to clean up after her one time! And this from a stray dog who may have never been house trained in her life!
The key to successfully house training a dog, no matter their age, is schedule and supervision. My parents had the scheduling part down pat – every hour on the hour. The supervision part is two-fold: supervision while you're home and supervision (or containment) when you're not at home, or unable to supervise.
My parents don't have a crate, but they are able to safely confine Copper when they go away from the house. They've got a tile hallway that's perfect for keeping a dog safe (off the counters and furniture) and easy clean up if Copper goes to the bathroom indoors.
As Copper and my parents adjust to one another, my parents will start leaving her alone with more freedom for short periods of time.
It's not hard to house train an older dog, it just takes patience, attention, and diligence!