It was retconned during D&D 3rd edition.
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.