An Ageable Entity is one that grows over time, like a cow.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.entity.IEntityAgeable;

IEntityAgeable extends IEntityCreature. That means all functions available to IEntities also are available to IEntityAgeable.

ZenGetter	ZenSetter	Type
ZenGetter growingAge	ZenSetter growingAge	Type int
ZenGetter ​	ZenSetter scaleForAge	Type bool

Methods expect an int and the second method an optional bool.
Both methods return nothing.
Normally, the second method does the same as the first with false as forced argument, that may differ for mod implementations, though.

ZenScript
 Copy
entAgObj.ageUp(int seconds, @Optional boolean forced);
entAgObj.addGrowth(int seconds);