68 seconds for men
78 seconds for women
Assuming you're talking about the general population. If you're only talking about track athletes, that's different. And if you're talking about general population with extensive training, that's different too.
And remember, most regular people peak in high school, before they gain their freshman 15 after graduation.
With no training, you'd basically have the track team guys, the cross country guys (mostly the same people), SOME of the soccer guys (no training, remember), and a few of the basketball guys, and maybe a couple/few of the football guys.
And remember, these days kids are fatter than ever, and more depressed than ever, and more sedentary than ever. 68 seconds is probably generous.