From Creatures Wikia
A life stage (or just stage) is a period in your creature's life, whether they are a norn, grendel, ettin or geat. Their needs will change as their life stage changes; some breeds, like the MerNorns, gain or lose abilities as they age.
The seven life stages are:
- Senile (Called Ancient in C3/DS)
There is a certain chemical (called Ageing in Creatures and Life in later games) which controls when creatures enter different life stages and ultimately die. An initial quantity of this chemical is injected upon creation, and the level decays from that point on. A series of genes is present to switch life stage at appropriate levels of the chemical, and eventually one of them triggers death. The change between one of these stages is called a life event. All genes controlling these transitions are subject to genetic mutation (or deliberate tampering).
The "die of old age" gene was left out of the original Creatures, including the Horse Norns, Brown Mouse Norns, White Haired Pixie Norns, Purple Mountain Norns and the Santa Norn released in 1996. Steve Grand says in his Creation book that this was accidental, and that his intention all along was to have forced euthanasia.
If you wish a creature to live longer, you can inject it with some more of the appropriate chemical. It will not grow visibly younger, but the transition to the next life stage (or death) will take place later than it normally would.
Most sprite sets - including those shipped with the games - do not contain separate sprites for each life stage. In the case where there is no sprite set for the current life stage, the previous set is used.
Life Stage Timeline (C3/DS)
This chart shows the ages at which a creature in C3/DS advances to a specific life stage (and at what age the creature will die of old age). These numbers assume that the creature is kept in good health, has not been exposed to any agents that may affect its Life, and is of the first generation.