In the American Civil War, you could get Generals in their mid 20s such as Wesley Merritt and George Custer and 20 year old Galusha Pennypacker. In the American revolution, Marquis de LaFayette was a general at 19. I guess in WWII the Americans wanted more experience (although Merritt and Custer were effective, George had problems a decade).
Edward Timberlake was the youngest American brigadier general at age 33 and he died in 1990. Dean Strother (35) died in 2000 and Garrison Davidson in 1992.
Apparently the youngest American admiral ever is Elmo Zumwalt who made it age 44 in 1964.