Mob Age as a Parameter for Resource Packs

If at all possible, I think it would be great for resource pack makers if the age of a mob could be used as a parameter to override the model/texture of a mob, just as it's name and other data can.
With this change, resource pack makers could add any number of "middle stage" models to the baby mobs to have a more granular visual representation of their growth, or even keep track of the oldest sheep in their herd by adding a model that sheep use after a few days, and mini game makers could make games that require you to keep a fish alive until it changes color.