Season 7 Episode 03 - Fragile Balance

A young teenager arrives at the SGC and tries to enter using Colonel O'Neill's pass card. He insists that he really is Jack, and that he just woke up small this morning and can't explain it. Grown-up Jack can't be found anywhere. Young Jack's DNA tests out positive and Jack starts to remember the Asgard doing some kind of experimentation on him, and Daniel finds computer records of alien abduction files matching Jack's account - the witnesses all have stories similar to Jack's, but without the deaging. And Dr. Fraiser discovers Jack's genetic structure is breaking down.

Jacob shows up to offer Young Jack the option of freezing, and Jack escapes. Sel'mak figures out that Young Jack is a clone and the team determines that an Asgard made the clone. Young Jack agrees to help them intercept the Asgard beam and Young Jack gets replaced with the real Jack. Young Jack has a zat gun and knocks out the Asgard, Loki, and beams SG-1 up to the ship. Loki reveals he was let go from the Asgard scientific council because of unethical experiments, and has been trying to perfect cloning to save the Asgard race and use O'Neill because his genetic structure is more advanced (see "The Fifth Race"). SG-1 contacts Thor who takes Loki into custody and manages to find a cure - Young Jack ends up going back to high school.