According to the USATF website, Derek Turnbull of New Zealand has the 65-70 World record at 4:56.4 on 2/29/92. He was 65 at the time, so someone between age 65 and 70 may have also run between 4:56 and 5:00. Depthwise, I know sub 5:00 after 50 is not that uncommon, and I personally know about 10 people, one being my father, who have done it.
As far as training, my father didn't do anything special. He just did his usual training (he was definatly not a miler, 2:16 marathoner in his prime), and then would just do a couple of speed workouts (ex. classic 8-10x400m with 1:00 recovery) before his one yearly mile race. He wasn't trying to run sub 5:00 every year like you though, and since he only ran this one mile race a year if he missed it he'd be screwed.