It looks like Alistair Cragg, who ran 13:03.53, has the second fastest 5000m time for a Caucasian after turning 30. Dieter Baumann ran 12:54.70 at age 31 (almost 32). Alberto Garcia, a Spaniard, ran 13:02.54 at age 30, but I do not believe he is Caucasian. Antonio Pinto of Portugal ran 13:02.86 at age 32, but I do not believe he is Caucasian, either. Perhaps this will give Galen Rupp something to shoot for, as his days of breaking 13 appear to be over, and he will turn 30 next May. Next after Cragg comes the late Stephane Franke of Germany, who ran 13:03.76 at age 31. Both Baumann and Franke were widely considered to have been dopers. If we don't count Spaniards and Portugese, it looks as if the next one after Franke is Dathan Ritzenhein, who ran 13:09.53 at age 30.
Interesting to note that Ben True ran 13:02 at 28 and 13:06 at 29.
Corrections welcome if I have erred in any way.