Most Bilatral animals have:
- Eggs
- a Larval phase
- Metamorphosis
- a Juvenile phase
- Growing Larger and Developing Sexual Organs (Puberty)
- an Adult phase
The Amniote evolved to have a longer Egg phase in lieu of Larval and Metamorphosis Phases. Most Mammals and a few other animals evolved Pregnancy to replace the Egg phase.
Jellyfish life phases are more complicated, and some Comb Jellies have the ability to repeat earlier phases of their life and regrow up. Lobsters, several Reptiles, a few Fish, and Trees never stop growing bigger (it has been suggested that some Sauropods were the same way). Growing animals often fill different niches at different sizes, meaning “Growing Larger and Developing Sexual Organs” could actually be multiple stages depending on how much bigger they grow.
I could understand if reverting to childhood or growing eternally are not doable (though I hope they are), but that is still a minimum of 6 Life Stages for many Earth Bilaterans (though, to be fair, for many Bilaterians, Metamorphosis is very short (a few days), and might not need to be played through). I also hope (and have suggested before here) that the player can continue to play as the parent until the egg hatches. But that would still be 4.