Jonathan "Jack" O'Neill is a clone of Colonel Jack O'Neill created by a rogue Asgard named Loki, to replace him while the real Jack O'Neill was being studied. Unbeknownst to Loki, the Asgard put safeguards in O'Neill's DNA to prevent genetic manipulation. This resulted in the clone only maturing to the level of a teenager while retaining all of the original's memories, so that Stargate Command initially assumed he was the original Jack, who had been reverted to youth for reasons unknown. During their investigation into what had happened, the young O'Neill recalled seeing an Asgard, prompting Dr. Daniel Jackson and Teal'c to investigate similar reports of alien abductions, to try and find an explanation for his transformation.
Closer examination of his DNA revealed that the young O'Neill also suffered from genetic breakdown due to Loki's cloning methods, prompting them to call the Tok'ra for assistance; it was Jacob Carter/Selmak's examination of the young O'Neill's DNA that revealed his clone status. With this knowledge - and the information acquired from questioning previous abductees - SG-1 were able to rescue the original Jack, capture Loki and contact the Asgard. Before Thor departed with Loki, Jack asked the Asgardian to help his clone, so Thor fixed the young O'Neill's genetic defects to allow him to age at a normal rate. The young O'Neill (being assisted financially by the Air Force) was last seen attending high school to get a second chance at childhood. The two O'Neills agreed that they wouldn't keep in touch because it would just be "weird", although the colonel told his clone to contact him in an emergency. (SG1: "Fragile Balance")