Three words: Little Big Horn.
Custer had a highly successful 15 year career before making a number of mistakes that led to his destruction at LBH.
My point was that some men have been promoted to very high rank at a young age and been quite successful, not that any such man never lost a battle.
If you want an example of a very young man promoted to the highest possible rank who never lost a battle, try Alexander the Great. To be frank, he promoted himself, but then he also never lost a battle and conquered just about the entire known world of the time.
Another example is Charles XII of Sweden, who had a highly successful military career for 8 years, starting at the age of 18. He conquered and damn near destroyed Poland and a good bit of Russia before being clobbered by Peter the Great at Poltava.
Scipio Africanus started his highly successful career in supreme command at the age of 25, his great opponent Hannibal assumed supreme command at 27. Scipio never lost a battle, and Hannibal lost only one.