5,000

I've run 5k's on the track and road since age 18 (I didn't count any trail or XC races) and this is a list of my best 5k by age:

By the numbers:

Age	Time
18	14:27.7
19	14:19.8
20	14:12.1
21	15:03.0
22	14:15.6
23	14:08.0
24	14:36.0
25	14:17.0
26	14:25.0
27	14:44.0
28	14:36.0
29	14:30.7
30	14:34.0
31	14:44.0
32	14:54.0
33	16:02.0
34	14:46.0
35	15:17.0
36	15:02.0
37	15:33.0
38	16:03.0
39	16:03.0
40	16:53.8
41	16:28.5
42	17:21.0
43	16:28.2
44	16:27.0
45	16:30.0
46	16:35.0
47	16:34.0
48	16:53.0
49	16:48.6
50	17:11.6
51	17:00.5

 Graphically