Mob aging with BetterModel

Mob aging in Minecraft

In Minecraft, mob aging is controlled by an int variable called age, first introduced in the abstract class AgeableMob and inherited by classes like AgeableWaterCreature, Animal, and AbstractVillager.

The age variable represents the number of ticks since the mob became an adult. Negative values mean the mob is a baby and is still growing up, while positive values mean the mob is an adult and is aging.
When mobs are bred, their age is usually set to -24000 ticks (20 minutes to grow up), which can be accelerated by feeding them their breeding item.

The problem

Mob stats like health and damage are managed by the server, but the size of the mob is handled by the client.
If you don’t use BetterModel, the client will display the backing entity in its small (baby) variant. However, when using BetterModel, the mob’s size will not change automatically—the mob will always appear the same size.

The solution

You can manually handle the mob’s size based on its age by overriding the setAge method:

Kotlin
Java

override fun setAge(age: Int) {
    super.setAge(age)
    val scale = attributes.getInstance(Attributes.SCALE)!!
    scale.baseValue = if (age < 0) 0.5 else 1.0
}

@Override
public void setAge(int age) {
    super.setAge(age);
    AttributeInstance scale = Objects.requireNonNull(
        getAttributes().getInstance(Attributes.SCALE)
    );
    scale.setBaseValue(age < 0 ? 0.5 : 1.0);
}

This approach overrides the setAge method of the AgeableMob class and sets the mob’s scale attribute based on its age.
If the mob is a baby (age < 0), its scale is set to 0.5; otherwise, it is set to 1.0.
This way, your custom animals will visually grow up just like vanilla mobs, even when using BetterModel.