Age 46 and my last 100 took me a little under 6 hours so average speed of about 17 mph.
It impresses my non-cycling friends when I tell them I've cycled 100 miles but to be honest I'm not that fit and it's just a case of spending time in the saddle and gradually increasing your ride lengths.
When I first started I thought 20 miles was a long way, now the only thing that limits my distances is the amount of time I have to spend on the bike.