The three-domain system of classification is based on differences in the cellular structures and evolutionary relationships of organisms, not how long they have been around.

Here's a breakdown:

* Three Domains: The three domains are Bacteria, Archaea, and Eukarya.

* Cellular Structure: Each domain has distinct characteristics, like the presence or absence of a nucleus and other organelles.

* Evolutionary History: Scientists use molecular data, like ribosomal RNA sequences, to understand the evolutionary relationships between organisms.

The three-domain system doesn't focus on age, but on fundamental differences in cell structure and how these differences reflect evolutionary relationships.