***Spoilers for The Rings of Power Ep. 6 ahead***
A fascinating detail - though whether or not the showrunners were aware of this, or can say as much with the rights issue, is up for debate.
Episode 6 of TRoP revealed Adar to be one of the original Elves to be tortured and corrupted by Melkor, which is commonly accepted to be the origin of Orcs. This is described in Morgoth's Ring, and the date given as between YT 1080 and YT 1085.
The interesting thing here is that this happened only about 300 solar years after the Elves first awoke in Cuiviénen. Quite possibly, though it is never specified, some of the unfortunate Elves that were turned into the first Orcs were among the original 144 Elves to awake. At the very least, the Elves tortured by Melkor were no more than a generation or two removed.
So what does this imply? Whatever the specifics, Adar is extremely old. As a member of one of the first generations of Elves to exist, not only would he be possibly the oldest physical being in Middle-earth by the Second Age, but it is conceivable for him to be one of the first hundred beings to exist in Middle-earth ever, older than even the Ents. Thus, in the film/television continuity (with the exception of wizards, Balrogs, and possibly Treebeard) he is older than any other character seen on-screen.