These are all "when they really were teenagers" fics:

There's Resonant's Experience (http://community.livejournal.com/sga_flashfic/353935.html).

Kellifer's In Passing (http://community.livejournal.com/sga_flashfic/206118.html) is another high school fic.

In notpoetry Campfire Law (http://www.talkoncorners.net/day25b.htm), they're teenaged camp counselors.

Chaps1870's Butterfly Effect (http://www.chaps1870.com/butterflyeffect.htm) is Gen, but there's a wonderfully strong connection between John and Rodney.

Ditto Nin's Hands-on Science (http://community.livejournal.com/sga_flashfic/207546.html).

Elayna's High School Confidential (http://furholt.net/elaynas_den/_McShep_HSCJohn.htm) takes place in a BDSM AU. (More here (http://furholt.net/elaynas_den/_McShep_HSCJohn.htm), here (http://elayna88.livejournal.com/462687.html), and here (http://elayna88.livejournal.com/461391.html))

And I can't recommend highly enough smittywing's Can't Buy Me Love fusion, The Best Things in Life Are Free (http://www.offpanel.net/smittywing/stories/tbtilafcover.html)