This hack is for Nightlife, and should only be used if nightlife is the latest expansion you have installed.
Versions for the other expansions are available from the download post.
The 3 variants of this hack should only be used one at a time:

NoAgingForAdults_NL.package Prevents Aging in adults.
NoAgingForAdults&Elders_NL.package Prevents Aging in both adults and elders.
NoAgingForElders_NL.package Prevents Aging in elders