Everybody is missing a major omission here.
Under 21:
#1 Gerry Lindgren 13:20.9 (1966)
Lindgren ran a 3-mile race on a dirt track in windy conditions in May, 1966 while 20 yrs, 60 days old in 12:53.0 T&F News has on numerous times converted this to 13:20.9 or 13:21.0 for use in their all-time lists.