I am between 8 when I am pulling my kids in the trailer with my heavy mountin bike, and 11 on my schwinn hybrid this year. Last year, I did the Ohio Tour de Donut, and for those aprrox 30 miles, I did have an average of 12.6 mph, but that was my last real ride before winter.
Mind you, I weigh about 235 pounds, and I don't have a real rode bike, but the hybrid with road tires and mountain bike style handlebars.