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?
Sources
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:
Best Answer
It was retconned during D&D 3rd edition.
D&D 5e's Volo's Guide to Monsters, p. 120, states:
This was already the case in D&D 3rd edition. Races of Faerûn (2003), p. 151, specifies:
This is a change from Dragon Magazine #151 (Nov 1989), The Ecology of the Yuan-ti, which originally stated:
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.