Same question, I've tried messing with the targeting in the ability def going so far as to use a modified version of the wilderspeakers EssenceTransfer and can't seem to get it to work on animals.

<VFECore.Abilities.AbilityDef ParentName="VPE_PsycastBase">
<defName>VPE_Age</defName>
<label>age</label>
<description>Very inefficiently siphons life force from a living target, de-aging the caster by 2 years and aging the other by 10. Very debilitating aftereffects for the victim and considered a hostile action by allies.</description>
<iconPath>Abilities/Chronopath/Age</iconPath>
<abilityClass>VFECore.Abilities.Ability_Blank</abilityClass>
<targetModes>
<li>Humanlike</li>
<li>Pawn</li>
</targetModes>
<targetingParametersList>
<li>
<canTargetSelf>False</canTargetSelf>
<canTargetPawns>True</canTargetPawns>
<canTargetAnimals>True</canTargetAnimals>
<canTargetMechs>True</canTargetMechs>
<canTargetBuildings>False</canTargetBuildings>
</li>
<li>
<canTargetSelf>False</canTargetSelf>
<canTargetPawns>True</canTargetPawns>
<canTargetAnimals>True</canTargetAnimals>
<canTargetMechs>True</canTargetMechs>
<canTargetHumans>True</canTargetHumans>
<canTargetBuildings>False</canTargetBuildings>
</li>
</targetingParametersList>
<range>3</range>
<castTime>180</castTime>
<castSound>VPE_Age_Cast</castSound>
<goodwillImpact>-25</goodwillImpact>
<showUndrafted>true</showUndrafted>
<modExtensions>
<li Class="VanillaPsycastsExpanded.AbilityExtension_Psycast">
<path>VPE_Chronopath</path>
<level>2</level>
<order>2</order>
<psyfocusCost>0.3</psyfocusCost>
<entropyGain>0</entropyGain>
<prerequisites>
<li>VPE_WordofForetelling</li>
</prerequisites>
</li>
<li Class="VanillaPsycastsExpanded.Chronopath.AbilityExtension_Age">
<casterYears>-2</casterYears>
<targetYears>10</targetYears>
</li>
<li Class="VFECore.Abilities.AbilityExtension_ExtraHediffs">
<onTarget>
<li>VPE_AgeSickness</li>
</onTarget>
<durationTimeOverride>300000</durationTimeOverride>
</li>

Age is hardcoded to ignore animals, since draining age from animals was deemed too OP.

No much different from draining animals vs draining prisoners / slaves

There's a lot more animals on most maps than humans.

Raids makes it so that you will always have a supply of human.
As well as generally living longer.

The only side effect of using it on animals is rapid aging, which more of a requirement then an exploit as some common mods such as Decent of dragons with dragons needing 20 years just to reach maturity.