Chip & Dale Rescue Rangers (1990)

Note: I did not beat an action RPG when I was as of yet, to be born.

Maybe there's a game older than that I've bested, but nothing comes to mind.

Edit: Owned currently? Pffh, I'm not even going to try.

The oldest game I own that I've completed is Maniac Mansion (1987), I completed the C64 version back in the day, long before playing the PC versions available here on GOG. :)

As for the oldest game that I don't own... I don't know, some Commodore 64 game for sure, though many of those old games had no ending, they just repeated in a loop. I finished all levels on Aztec Challenge (1983) but it never ended, it just started again from the first level. I also always defeated the computer on Archon (1983). Those are the oldest I can remember.

It's probably Ghosts 'n Goblins (1986).

Colour me impressed... that one's really hard.

To be honest, it wasn't even fun beating it. When finished, I didn't even get any satisfaction from it, I just felt spiritually broken. The sense of achievement was more in line with being a puppy who endured getting beaten by the owner from room to room until the owner finally passed out.

Good day. My real favorite old game is the traditional cards and the variations on them.

This thread has just been Necro'ed from 2018.

... but it's still good.

The oldest game I have completed is definitly not the first game I have completed.

The oldest one is probably a point & click adventure. I'd nominate Indiana Jones 3 now, since I sadly never finished Maniac Mansion and Zak MacKracken.
Maybe some Sierra adventures which I played (and finished) are older.

The oldest game I played (but can not be completed) is Pong.

Yes, interesting thread. A helpful bot, for once!

The two oldest games for me was the original Broken Sword (1996 game) and Earth 2140 (1997 game). Both completed during the last two years.
I can't think I have even played anything older.

Space Quest (1986)

I played many older games but haven't finished them yet. :-/

May have beaten older games since, but the first computer game I completed must have been Bruce Lee on the C=64.

Probably "Castle of the Winds". I think it is the first good roguelite, made in the era of Windows 3.1. The game had very simple controls, a thorough help system, graphics that weren't ASCII, the user being able to set up spells in a hotbar, mouse-driven, and so on. Only lacked sound and music.

Rick Saada is apparently starting work on a remaster. Got a response from him about a month ago, so things are looking good on that front.

Eye of the Beholder probably. Also might be oldest I actually played, not only beaten.

Funnily enough it was much more seamless experience I had than with many newer games from 90's or even 2000's. That's due to incredibly robust gameplay that doesn't really play any different today than say Legend of Grimrock.

EDIT:
I was wrong. Leisure Suit Larry parts 1-3 are older, so that's it. Original 1987 LSL.

Well, that game could actually be my answer to the original question. That one or Syndicate, whichever came out first.
As for games that I used to own... it's not that simple. I don't remember beating any game on the Atari 2600 (they were of the "endlessly looping" variety, or sport games that couldn't properly be "completed"), so it must have been one for the C64. Hard to tell which one, though (maybe Forgotten Worlds, from 1989?)

I've never been able to finish that, sadly...