I propose the addition of age tags, which apply tags to a unit when it reaches a certain age.
Before I get into the meat of it, some use cases. Baby birds can't fly, baby bugs are usually completely different in body plans to their adult instars, baby frogs/toads gain limbs incrementally and, this is a stretch, maybe hydras grow more heads when they go through hydradolescense.
The age tag would have three arguments: time in years, time in days, and whether or not the creature is completely healed when the age tag is applied (for animals that molt/pupate/transform). After the tag is the indented block of tokens to be added. Let's look at this giant butterfly.
[AGE_VARIATION:1:0:HEALS]
[FLIER]
[BODY:2WINGS]
[APPLY_CREATURE_VARIATION:insert cvar here that removes caterpillar/pupa specific anatomy]
When our butterfly reaches 1 years old, it gains wings and the ability to fly. That creature variation could be something that removes a jawed mouth and replaces it with a piercing proboscis. This example is pseudocode written on a napkin basically, but in brief:
Age variations basically add tags and/or apply creature variations when a unit reaches a certain age. From my experience the raw parser has no problems adding tags if they're properly formatted, or removing tags with creature variations, so this just seems like a natural extension of the systems we already have in-game.
This is about as elegant of an approach that I can think of to give proper life cycles, instars, molting and metamorphosis to animals that do so IRL. You figure most of what's going on in real life is just animals gaining or losing an ability, gaining or losing body parts.