I'm 50, 210 lbs or so, riding 8,000 miles a year. On my Raleigh Sojourn, I'll average between 15 and about 16.4 mph on a 30 mile open ride. This is the speed from my computer, which stops timing when you stop rolling. It'll be lower if there's lots of traffic or stop signs, "open" means very few stops required. There's older guys in my club that can ride circles around me, so that's not especially good. There's also older guys that are slower than me, for that matter. Going all out on a 20-mile time trial a while back, I got it a bit over 18 mph. I once did a 100k averaging 19 mph, but I think that was a tailwind-both-ways situation that I've never repeated.