As everyone says, there are many variables.
The first is the rider, the others are the conditions, the bike, the weather, clothes, etc.

I ride a mtn bike and have over 100,000 miles on it in 30 years.
I've commuted 50,000 miles on it in the last 7 years.
My avg speed is 13-14 mph, but I have 15 miles of gravel roads a day, I was 1+ mph faster back when I was on pavement only.
I also have a road bike which goes faster, but I do not commute on it.

YMMV