I do not think age has that much to do with it..Conditioning,training and genetics...I have known some pretty strong riders in their 60's and they had the body of a twenty year old...Around here , we have challenging hills with 2 mile long grades at maybe 8 %...That slows me down..
I am in your same age group. On flats I average about 15-18 mph , depending if I am fighting a 20 mph on shore wind.