Minecraft wiki entry for Breeding, associated text: Baby animals may also be spawned using the /summon command with a negative Age tag; for example, using /summon sheep ~ ~ ~ {Age:-100} spawns a baby sheep at the player's position, that matures in 100 ticks (5 seconds). For baby mobs that don't grow up like zombies and piglins, the IsBaby:1 tag is used instead.