Usually you are doing good if the tires are less than 6 months old, especially if you are buying tires in a common size.
If you want to accept older tires, ask for a discount to make up for the time lost. It doesn't hurt to ask and sometimes the dealer will give it to you to make the sale. If your trade-in tires are recappable, you may also ask how much they will give you as a carcass credit.
Here is what one place pays:
tire values