Irken Smeets are Irken babies and (later) older children, created in a large birthing facility known as "The Hatcheries". These are found beneath the surface of the Irken homeworld, Irk. Smeets are created by the Control Brains.
The Brains mix selected Irken DNA and fuse them together to make an Irken embryo. The embryo is placed inside a developing tube similar to a human womb. The time it takes for a smeet to fully develop is unknown. Seconds after they are "born", a PAK is placed on their back and they are "activated" with an electric shock.
Unlike human babies, Smeets can speak as soon as they are born, as seen in Parent Teacher Night. When they are two minutes old, they are given a name, a personality, and the whole of Irken knowledge (culture, history, geography, and the like), which is transferred into their PAKs (seemingly save for military knowledge, as they spend much of their early lives learning battle tactics). Like human children, they have few teeth (usually one or two) but when they hit growth they start to develop their full-grown teeth. Zim's smeethood was revealed in Parent Teacher Night. 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 smeets play with toys, like human children. These toys are described as advanced, but it is unclear what they look like.
About Zim's smeethood
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 explains why 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". 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 pasts. 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.