The 5e playable race states that they live about as long as humans in rare cases up to 120 years. But all the lore I can find on them says they live up to 2000 years. I am not too familiar with the D&D universe, but did they retcon it in 5e or something?
These are not really hard evidence since it is 3rd-party information, but these are the sources that suggest that the yuan-ti have a 2000-year lifespan:
D&D 5e's Volo's Guide to Monsters, p. 120, states:
Purebloods mature at the same rate as humans and have lifespans similar in length to theirs.
This was already the case in D&D 3rd edition. Races of Faerûn (2003), p. 151, specifies:
On average, yuan-ti live to be about 80 years old, although some have been known to live over 120 years. Yuan-ti have the same life expectancy and age categories as humans.
This is a change from Dragon Magazine #151 (Nov 1989), The Ecology of the Yuan-ti, which originally stated:
Pureblood yuan-ti age as do gray elves (AD&D 1st Edition Dungeon Masters Guide, page 13). Halfbreeds live twice as long as purebloods, and abominations have a life span three times that of a gray elf, some 6000 years.
Since the 1e DMG considers gray elves of venerable age at age 2,000, this suggests the typical lifespan of a yuan-ti pureblood in AD&D was 2,000; halfbreeds 4,000; and abominations 6,000. (As per DMG p.15, individual lifespan can vary.)
If I had to speculate as to why the change was made, it is possible that early Dragon articles were simply unknown to writers some 13 years later. Yuan-ti lifespan wasn't mentioned in any of the AD&D 1e, 2e, or 2e monster manuals, and the writers may simply have been unaware of one article from 1989. They might also have deliberately ignored it if they thought the numbers were too high. They may also have deliberately changed it to make it easier to play yuan-ti purebloods as player characters, something possible in third edition thanks to its level adjustment system, in which case it would be more appropriate to have them more humanlike in lifespan.
The yuan-ti first appeared in Dwellers of the Forbidden City which didn't mention their lifespans. However, they were given a comprehensive "Ecology of ..." article in Dragon #151 which says:
Pureblood yuan-ti age as do gray elves (AD&D 1st Edition Dungeon Masters Guide, page 13). Halfbreeds live twice as long as purebloods, and abominations have a life span three times that of a gray elf, some 6,000 years.
This meshes with your memory.
I won't go through all the subsequent changes but suffice it to say the age range for a yuan-ti purebred PC in Volo's Guide to Monsters is considerably less.
