Answer:
The BSA states " Not every boy who joins a Boy Scout troop earns the Eagle Scout rank; only about 5 percent of all Boy Scouts do so. "
In 2010 there were 898,186 Boy Scouts and 56,176 Eagle Scout awards presented, thus it would appear to be 6.2%. But male Venturers who earned First Class in a troop can continue to work towards Eagle Scout while in a crew. There were a total of 238,846 Venturers, but we don't know how many were eligible, but the percentage goes down.
As of 2008 (the last year I have totals), there had been 83,486,083 Scouts since 1911 and 1,880,905 Scouts had earned Eagle Scout, making it 2.2% for the total.