This seemed like a fitting thread to meet Lone_Scout's condition, looking through the very oldest still available outside archive. Could have also taken the one it was inspired by, but this allows for a slightly more thorough answer, right?

So, yeah, oldest game owned and beaten, and also oldest game owned, period, is also Akalabeth (1980), though it was rushed through, with the amulet trick.
Since that was free, oldest game actually purchased would be Might and Magic I (1986), but that wasn't finished. But in that same bundle is the oldest one purchased and finished, and that's Might and Magic VI (1998). Gather it was released earlier in '98 than Return to Krondor.
As for physical copies, since I gave away my copies of Planescape: Torment (1999), Soulbringer (2000), Gothic (2001) and Heroes of Might and Magic IV (earlier in 2002) and I didn't finish Morrowind (also earlier in 2002), guess oldest that I still own physically and have finished is the original campaign of Neverwinter Nights (2002). Divine Divinity seems to have been released a bit later in 2002. (Was that a good year or what?)

Looks like I win the thread. ;-)

The oldest game I own that I've beaten is Adventureland from 1978.

I believe I've glitched through Wizardry 1 (Apple 2 version) in record time.

Thing is:
* Getting a high level Bishop is easy, thanks to the identify glitch. (Identify item 9 (you can only carry 8 items) and you will get 100 million XP, allowing you to level past llevel 200.)
* Casting MALOR during battle will sometimes teleport you to level 10 of the dungeon. If you are really lucky, you will end up right outside Werdna's room.
* For the actual Werdna fight, the trick is to cast MAHAMAN, which I believe has a 1 in 3 chance of teleporting tbe enemies away, which counts as a victory. (Note: This works differently in Wizardry 2 or 3; in those games, you want to cast HAMAN instead of you want that particular effect.)

(Incidentally, the actual victory came decades after when my family owned that particular version of the game, I believe.)

Oldest I currently own, considering different versions of different games as separate, is probably Ultima 3.

I won a game of Pong in the mid 70's - does it count?

The oldest I can think of are The Secret of Monkey Island and Loom, both released in 1990.

Not entirely sure, but I think it's probably The Legend of Zelda (1986).

Probably Myst - I think I still have the original CD floating around somewhere. I played other games before that, such as the original Civilization, but those didn't belong to me.

In my case, it's Indiana Jones and the Last Crusade (1989).

en.wikipedia.org/wiki/Romance_of_the_Three_Kingdoms_(video_game_series)
(1985)

UPDATE:
en.wikipedia.org/wiki/Karateka_(video_game)
(1984)

The Fool's Errand(1987) on my little Apple II. I lost my original copy a few years ago, but luckily I'm able to still be able to find it on old Abandonware/Shareware sites as long as I'm willing to take a couple surverys. :P

If I don't currently have to own it, just have owned it at any point in time, then the 1977 Atari 2600 "Blackjack". And I think I beat "Breakout" (1977) unless I owned "Super Breakout" (1978) instead (memory gets a little fuzzy that far back).

And I actually beat "E.T." and "Raiders of the Lost Ark"!

The oldest game that I 100%'d was Spyro The Dragon (1998). A solid investment of time & dedication.

Pokemon Gold and Pokemon Red. Beat all gyms and Elite 4.

Drakkhen (1989). Got in early-mid 90s. True, I didn't beat it until ~2005 when the Internet helped me to solve the final quest.

Sid Meier's Pirates! (1987). I had the boot from 3.5" low-density floppy CGA version of it. Got in early-mid 90s ("Sid Meier's Triple Action Pack" that also had Railroad Tycoon and Covert Action). Open-ended, so no real "beat" to it, just ever escalating family members to rescue.

There were a couple text adventures from back around then I don't recall titles or plots at all, but none of the "big" ones.

I tried early Ultima games and am rubbish at them.

This excludes plenty of earlier console (NES) games.

Scott Adams's Adventureland, on the VIC-20. So, 1981, I think? That port might be 1982.

Edit: This was also the first game I beat on my own, without Dad helping. With Dad's help, the first game we beat was Adams's Mission Impossible text adventure.

We had the VIC-20 and some cartridges, but no tape or disk drive, so we had to beat the adventures in one sitting and take notes!