Basically, the title says it all.
What is the lifespan of a beholder ?
I searched in the 5E Monster Manual and Volo's Guide to Monsters but could not find any hints about their lifespan, so a response quoting older rulebooks is welcome.
I'm also interested to know when they start dreaming about their death and thus eventually transforming into Death Tyrants.
From the 3.5E Sourcebook Lords of Madness
In theory, a beholder becomes increasingly frail as it progresses beyond its ninetieth year. By the age of one hundred, its eyestalks begin to lose their powers and wither away. Only rarely do beholders live past the age of one hundred twenty. In practice, though, a beholder cunning and strong enough to survive attacks from powerful adventurers and other beholders finds a way to extend its lifespan with magic. If a beholder can stave off aging, it continues to grow slowly in size. Stories persist of immense beholders several hundred years old and nearly 20 feet in diameter.
Ed Greenwood's "Ecology of the Beholder" in Dragon #76 says one beholder lived for "over 900 seasons". My first assumption is he meant "years" instead, but even if we take it literally, with four seasons per year, that's 225 years. He also had them laying eggs that took up to a year to hatch, with the hatchlings reaching full size in a year.
