Smeets are Irken babies, created in a large birthing facility known as "hatcheries" (commonly referred to as 'smeeteries'). These are found beneath the surface of the Irken homeworld, Irk. Smeets are created by the Control Brains in the show, but in most fan works most are born naturally and/or are hatched from eggs resembling reptile eggs.
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.
Non-hybrid and hybrid Smeets can speak as soon as they are born, as seen in Parent Teacher Night (others do forms of communication like wiggling their antennae or purrs and clicks). When they are two minutes old, they are given a name, a personality, and the whole of Irken knowledge, which is transferred into their PAKs (seemingly save for military knowledge, as they spend much of their early lives learning battle tactics), though hybrid Smeets usually have childhoods similar to those of 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 the episode Parent Teacher Night.
There on after for a ten year period they spend their childhood dwelling under the surface of Irk, being trained in combat and warfare tactics. After this decade period is up they can be referred to as an irken and are encoded jobs and ranks to match his/her DNA abilities and talents, but most importantly their individual height predetermines their class distinction in Irken Imperial Society.