As long as there is no sign of impending failure and you don't do anything to hide the age, it's on the buyer to ask about it.
Now if there is a bulge on the inside of a tire that you are aware of that is the sign of impending blowout, that's a different story. Now you would be putting the buyer at risk but even then, I would point it out after the buyer makes an offer and let the buyer research the price.
For all you know, the buyer will drive it 10miles down the road and park it never to move again, so he could care less about having new tires.