Really depends. Seemed Avis would get more newer cars in the fall, historically. But the pattern seems to have minimized as they're getting new cars all year replacing high mileage (hitting their tier limits), damaged cars or to meet local need.
Also, it depends on when the 2018 models come out. Some are out in the spring of '17 while others are closer to 2018 so that will depend on when you can find the '18 of a specific model.