I'm 66 now - first century was when I was 58. I've never come close to 20 mph - that's way out of my league. The fastest average speeds I've had (three different rides at approximately the same course - from central Massachusetts up to Brattleboro VT, across the CT River to NH and back down to the starting point) were 16.3, 16.4 and, last year, 15.6 mph. About 3,600 feet of climbing - rollers throughout but no big hills - about as flat a century as you can find in New England.