Irken Smeet is the term commonly used for Irken children and infants. Despite the fact that all modern smeets are created underground, in large unnamed facilities on their homeworld of Irk as clones, it's very likely that the Smeet has changed very little over time. Notably, smeets are created by Control Brains.
Creation and Birth
Although the exact methods used in the creation of smeets are vague, it is widely accepted that the Control Brains mix either selected or randomized DNA and fuse them in order to create an embryo. The embryo is then either placed within a capsule for gestation, or was in the capsule in the first place, staying within the presumably life-sustaining green fluid within as it grows. Notably, it is unknown how long a smeet gestates and grows for, though it is very likely the Control Brains or the capsules themselves will indicate when it has reached sufficient maturity to be born.
Upon sufficient maturity, the holographic face on one end of the capsule turns green and smiles. Within the hatcheries found beneath the Irk's surface, robotic arms unscrew the capsule from a wall filled with others, steam emitting from the formerly-filled space in the wall. This is a very likely indication that the chambers the capsules sit in are heated, likely to keep the smeet warm before birth.
Following removal of the capsule from the wall, the robotic arms haul the capsule over the tiled ground, and flexes it upwards. Notably on the capsule, there is a hinge in the middle between each end of the capsule, likely for re-usability or safety as the glass shatters, the smeet and fluids pouring out. The smeet then flops onto the ground, and two robotic arms swing down, one with a PAK.
The PAK-wielding arm acts first, pressing two heated spikes (likely to cauterize the inflicted wounds) into the smeet's back and attaching the PAK immediately after, likely connecting to the spine more easily through the holes. The other arms comes down after as the other retreats into the ceiling, and the smeet is activated with a presumably-electric shock and welcomed to life, with orders to report for duty. Zim's birth was revealed in Parent Teacher Night, and is also the source of much of the information regarding birth.
Unlike human babies, Smeets can speak as soon as they are born, as seen in the depiction of Zim's past in "Parent Teacher Night". When they are about two minutes old, they are given a name, a personality, and the supposed whole of Irken knowledge (likely culture, history, geography, and the like), which is transferred into their PAK (seemingly save for military knowledge, as they spend much of their early lives learning battle tactics). Like human infants, they have few teeth (usually one or two) but when they hit growth they start to develop their full-grown teeth. This part of Smeethood can decide where they will end up being in the Irken Society, due to the fact that their Irken Knowledge can vary and can be Defective or Affective.
It was mentioned by Zim in "Bolognius Maximus" and "Halloween Spectacular of Spooky Doom" that Irken babies play with toys, like human children. These toys are described as advanced (even more than Professor Membrane's lab technology), but it is unclear what they look like.
Upon receiving data in the birthing facility when he was born, Zim complained that he wasn't given enough, and that there were many mistakes in his PAK's programming ("40 schmillion", to be precise), which is likely the reason he is a Defective.
Over the course of their childhood, smeets are trained underground in military tactics for ten years until they are old enough to be referred to as Irkens in full. Notably, Zim's first words were: "I love you, cold, unfeeling, robot arm!" when he embraced the arm of the Control Brain that welcomed him to life.
The cancelled episode "The Trial" would have shown more scenes from Zim's smeethood, as well as more on some of The Tallest's past. These flashbacks prove that Zim and Red and Purple actually trained together, and that Zim has been causing planet-wide chaos since he was literally three minutes old.