Working good old and game into a title was not quite as easy as I expected, without turning it into a convoluted or lengthy mess.

So the question is: In general, how high is your tolerance for old games? Is it different for different genres? Do you have a specific break point, past which (most) games are night unplayable? And how close is this break point to the time when you first started playing games?

For me, it varies quite a lot between genres.
Console platformers are usually fun quite good, even if they are old, although I can't think of any outstanding ones that pre-dates the 8-bit era. I started playing these types of games during the 8-bit era (1989, to be more precise)
For vertical or horizontal scrolling shooters, I generally find 16-bit games or late 80's arcade games to be quite playable, while 8-bit games are less than stellar. Much like platformers, I started playing these during the 8-bit era.
For CRPGs, the break about is roughly 1990, with most 80's CRPGs being too focused on combat and grind, and lacking story. Games like Ultima 7, Might & magic 3, Eye of the beholder and such are still quite playable. I started playing CRPGs around 1998-1999.
For shooters, I found it hard to play games that pre-dates Quake. The controls usually feel to awkward. When i first started playing these, I had no problems with Wolfenstein 3D, but these days I do. I started playing shooters in 1997.
For strategy games, Battle Isle is roughly as far back as I'm willing to go for TBSs and for RTSs it is Command & Conquer. I started playing RTSs in 1997 and TBSs in 1999.
And for Adventure games, mouse support is the key issue, without it I find them frustrating. So around 1990 for these as well. I started with Shadowgate for the NES in 1991.

FPS - it's got to have proper mouse and keyboard support. Build engine games just about scrape by (aided by the fact that some are fantastic). But not Doom era. Not anymore.

RPG - Not too picky, but usually I don't play anything older than the infinity engine games now.

Racing - I don't think I'd be inclined to play anything more than seven years old now.

Strategy / Tactical - nothing too low res and / or unclear. Clunky controls or interfaces are usually a deathblow too.

Adventure - early 90s.

For me, as far as PC gaming is concerned, I guess it would be about 1990. I think I'd be reluctant to play any games that only run in EGA or worse CGA or use only PC Speaker sounds. Early 90s VGA games with Soundblaster support are still cool though, IMO. There might be a few exceptions to it, but generally I think that's a good rule of thumb for me.

Then there's that period in the mid 90s when a lot of devs were abandoning appealing 2D VGA graphics to experiment with crude new 3D looks - that's another period I'd be a bit wary of. I was also never really fond of FMV games; I have somewhat changed my mind in that regard, they can actually be fun, but they still look terribly cheap and dated most of the time, also due to the horrible fashion style of the 90's. ;)

So I'm very tolerant, even admiring of 90's 2D VGA graphics and also colorful pixel art, not that forgiving towards (by today's standard) crude 3D graphics and low quality videos.

(This only for PC, I'd probably be less strict for C64, NES, SNES, Amiga or Atari ST games. I think I got into gaming about '86 or '88 or so, so quite close to my personal PC boundary.)

It's also quite close to the time when IBM PC gaming actually took off and the system started seeing more than a dozen new games a year.

Same here, to a large degree. In particular in regards to crude 3D graphics. The awkward phase of 3d graphics, when polygons was just becoming a household word, resulted in a lot of games that has aged poorly

I was recently pleasently suprised to find that the original NES Zelda was still totally playable
I don't think there's much else I'd play from the 8-bits these days - possibly Boulderdash if I hadn't played it to death already, the orginal Pokémon games on the original GB I would image could still be fun for a bit
I could imagine there's Amiga era stuff that's prolly still fun - I keep feeling tempted to dig the original Wing Commander out...

I'm much more forgiving to 2D, isometric graphics in old games because they age so much better, and in my opinion are more creative. In games like Baldur's Gate and Fallout the art is often only as good as the artist who draws the background "areas" or screens or whatnot. I still find the pixelated backgrounds of the first Diablo aesthetically pleasing, for instance, but a 3D game of the same vintage, or even a few years younger, is quite hideous to me much of the time. Another reason why I miss the golden age of isometric cRPGs so much.

In general, my cutoff for older games I have not yet played is about 1996/1997. Before that, especially if I didn't play it before in the distant past and thus nostalgia is more forgiving to its defects, the graphic/sound and GUI culture-shock is typically too much to absorb.

However, with games like Betrayal at Krondor, Ultima VII, and WC 1/2 that I played growing up I can happily revisit. Other games that passed me from the same era that many GOG-ers gush about unfortunately I won't be able to get into as a neophyte. This much I *know* about myself, despite protestations of others.

For RPGs, I can't really handle anything that's older than about 1990. But the mid-late 90s were the golden era for RPGs, for me at least.
For FPS, Doom-ish or later. I still play Doom, Dark Forces, Heretic, and Hexen periodically.
For puzzle games, any age is good. Things like Breakout, Tetris, and Space Invaders are still extremely fun, and they're 30+ years old.
For RTSes, I can't handle anything before Homeworld. I find most of the earlier games to be really clunky or lacking in features. For instance, StarCraft handles armor/damage reduction in a silly way, and everyone always goes in single file, which bugs me. It's hard to go back to StarCraft after playing Dawn of War, for instance, but Homeworld is still good.
For TBSes, basically anything works as long as the game mechanics aren't terrible.

90's.

But for RTSs - Warcraft 2 - before that it is painfull. Also those first 3D rts around 2000 are out, because they look so... minecrafty.

I (*gasp*) never liked Pac-Man much, it was supposed to be a bit of a churned out 'B-Movie' release for Namco that year, their blockbuster was supposed to be 'Rally X' which I think is actually a better game, you can't argue with the iconic design of Pac man (originally 'Puck man' because he looks like a hocky puck) and the ghosts though...

Flashing back to older games I find two main thresholds for me. When we are talking 2D then I feel that SNES games have aged very well, but there aren't many NES games you could talk me into playing. 4-bits just isn't enough color. When it comes to PC games the old mode x 320x240x256 resolution games I would still play. I still itch for a round of One Must Fall from time to time.

When it comes to 3-D games, I can not handle the crude early days of the Playstation 1 / Saturn era. I felt at the time that 3-D was being rushed out the door a little too soon on the console side, and looking back I wont go back there. Its visually painful. PS2 and Dreamcast fixed all that and I am more than happy to play some of those games. Soul calibur arcade - no. Soul calibur DC - yes.

Oldest game I own of GOG's catalog is Duke Nukem 3D (1996).
I play through most if not all the 8-bit Mega Man games every 2 years or so, as part of the Mega Man Anniversary Collection on the PS2. They're the best, best, best. They'll never get old, and the music...man, oh man, the music.

For me, its around that point where EGA/VGA graphics started being used more frequently (and the use of the mouse also helps). So i guess from around 1987 to 1995 is the time period i target. EGA games and lack of mouse support are not so much of a problem for me with RPGs or Adventures - i don't have a problem with, for example, Space Quest 2, Wasteland, Battletech Crescent Hawks Inception, Pool of Radiance, Curse of the Azure Bonds. Most games made before 1987 I find are hard work.

i am trying to get into games made after 1995, but i find i tend to buy, download and then go back to older games, but hopefully that will change when i catch up on my old games back log.

Doesn't matter to me.