To get around the rule against perpetuities (mandating no dispositions could go beyond 21 years after the death of persons living during enactment - IANAL, please), lawyers uses the lives of a defined set of persons. Already in the Deuteronomy, the lifespans of the High Priests were used to determine whether someone in a city of refuge (a TIL in its own right) could go back home. There's also tontines (another TIL).

In some common law jurisdictions, the lives of the British royal family are used because they are a well defined set (no ambiguities about secret members) and their members enjoy a ligh level of life and healthcare (ensuring long lives - see how long lived Elizabeth II with the stress of an official position).

Other states use other figures such as Presidential lives or patrician families (Rockefeller and Kennedy) in the USA or Eamon De Valera's family in Ireland.