So I don't get too excited by the occasional developer suggestion for changing the appearance of a mob (whether a creature or an NPC) based on that mob's age. It would be a minor addition to the worldiness of the game, so I wouldn't object to it, but it wouldn't excite me.
What would excite me would be if the behavior of a mob changed according to the maturity of an individual mob, the normal intelligence level for that type of mob, and the environmental conditions.
Assuming a world with Juvenile, Mature, and Aged versions of a mob, it's fun to imagine them displaying different behaviors in different circumstances. Maybe something like:
|Juvenile||play||run to parent||defend||escape|
|Juvenile||investigate||attack||run to parent||escape|
|Juvenile||play||investigate||run to parent||escape|
(Note that this is primarily for combat interactions. For other kinds of activities, semi-intelligent and intelligent mobs -- NPCs -- would have other tables for generating other kinds of reactions in other situations.)
If distinctive behaviors like these were implemented (especially with a little bit of randomness, and with the ability for members of groups to signal their states to each other), the game world would feel vastly more dynamic.