A fictional event created by Robert A. Heinlein.
Forseeing bigotry and persecution on the part of those who were not part of the Howard breeding experiment and thus not eligible for the Howard's exceptional longevity, the Howard Foundation instituted a policy where each member of the Families would change identities every several years. This way, the actual age of the individual (which could potentially be several decades above the apparent and assumed ages) would not be ascertainable. It was this policy that led Woodrow Wilson Smith to adopt the nom de guerre of Lazarus Long. The theory behind the Masquerade was validated when the Masquerade was ended and widespread persecution of Howards did indeed take place.
The Masquerade includes events in both To Sail Beyond the Sunset and Methuselah's Children.