The roof of our house is 26 years old. It is guaranteed for 30 years. It is showing signs of wear. I might says "We only have about four years left in this roof." A phrase like "[time] left in" X means that we are imagining health, or strength, or good functioning as being something "inside" X, something that is gradually lost over time. Knees get weaker with age. They only have a certain number of years "left in them."

Could he need knee replacement surgery perhaps? 🤔