Here is how I see this - the age of entrepreneurs depends on level of expertise and previous experience needed to run a business:

- for Web / consumer startup the majority of entrepreneurs are in 20s

- for enterprise enterprise applications startup is seems like entrepreneurs in 30s

- for enterprise infrastructure startup is seems like entrepreneurs are in 40s

As of now the most of growth is in Web / consumer startup so it seems like all entrepreneurs needs to be in 20s.