I am watching the TOS movies in a row right now, and noticed something very strange in The Voyage Home: to get some money, Kirk sold the glasses that Bones had gifted him in an antique store. So Spock asked "Admiral, weren't those a gift from Dr. McCoy?" and Kirk responded with "And they will be again, that's the beauty of it."
Now, IIRC, in The Wrath of Khan, Bones said he got those glasses at an antique store. This seems to strongly imply that after Kirk sold the glasses, they lie in the store for 300 years, then Bones picks them up and gifts them to Kirk, they travel back in time, and Kirk sells the glasses to the antique store, they lie in the shop for another 300 years, before Bones picks them up and the whole cycle starts again. This would mean that these glasses are eternally going through this cycle, and thus are infinitly old (and have never been assembled).
Now, IIRC, in The Wrath of Khan, Bones said he got those glasses at an antique store. This seems to strongly imply that after Kirk sold the glasses, they lie in the store for 300 years, then Bones picks them up and gifts them to Kirk, they travel back in time, and Kirk sells the glasses to the antique store, they lie in the shop for another 300 years, before Bones picks them up and the whole cycle starts again. This would mean that these glasses are eternally going through this cycle, and thus are infinitly old (and have never been assembled).