Best American and World 5000m times by Age:
18
World best Eliud Kipchoge 12:52.61
German Fernandez 13:25.46
Chris Derrick 13:29.98 (tied for #5055 performance all time)
19 (world best Eliud Kipchoge 12:46.53):
Dathan Ritzenhein 13:27.77
Hassan Mead 13:28.45
20
World best Daniel Komen 12:45.09/#2 Stephen Cherono 12:48.81
Bob Kennedy 13:22.17
Evan Jager 13:22.18
Alberto Salazar 13:22.31
Rudy Chapa 13:25.57
21
World best and WR 12:37.35 Kenenisa Bekele/#2 Daniel Komen 12:39.74/#3 Sileshi Sihine 12:47.04
Bill McChesney 13:18.6
Rudy Chapa 13:19.22
Jim Hill 13:19.73
Josh McDougal 13:20.43
Steve Prefontaine 13:22.8
Paul Geis 13:23.38
22
World best Haile Gebrselassie 12:44.39
Alan Webb 13:10.86
Chris Solinsky 13:12.22
Bob Kennedy 13:14.91
Bill McChesney 13:15.77
Ryan Hall 13:16.03
Galen Rupp 13:18.12i
Dathan Ritzenhein 13:22.23
Alberto Salazar 13:22.6i
23
World best Bekele 12:40.18
Bob Kennedy 13:02.93
Alberto Salazar 13:11.93
Bill McChesney 13:14.8
Ian Dobson 13:15.33
Dathan Ritzenhein 13:16.61
Brent Vaughn 13:18.46
Steve Prefontaine 13:21.87
24
World best Geb 12:41.86/#2 Bekele 12:48.09
Bob Kennedy 13:03.37 (2 days before 25th birthday)
Matt Tegenkamp 13:04.9
(didn't check for BK's best at 24)
Adam Goucher 13:11.25
Jim Spivey 13:15.86
Dathan Ritzenhein 13:16.06 (Brasschaatt 2007)
25
World Best Geb 12:39.36
BK 12:58.21/12:58.75 (nearly 26)
Obviously, take the ages of the world best age group guys with a grain of salt, as a number of those guys never got better, so they were probably years older.