Reading.
The gist I get is a "basic" model of creature. Additional XP is only gained by "age" of the creature.
No problem. Have standard "age" ranges such as Newborn, Young, Juvenile, Early, Elder, Middle, Old, Ancient.
All creatures go through these stages, especially mammals, reptiles, amphibians, birds, et. al.
Insectoids deviate greatly in having Egg, Larval, Nymph (if), Adult.
Mollusks may also deviate. Protozoa definitely do. Protozoa are basically binary. They exist or they don't.
Companion "creatures" DO earn XP. Regardless. Some can even learn certain things to do when a certain "thing" is done, such as a tongue click followed by a certain whistle. The companion animal then does those "things" trained to do.
Some can even learn "spoken" commands, such as "regaltha" meaning to "take point and protect."
Thus, yes, give us a table showing a more divisional separation between the "age" of a creature; rather than, giving us a "how to build a creature from scratch." Add that in a supplement.
rmfr