Napoleon and his generals all rose to positions of power while still quite young:
What makes them seem so young for their rank is our own career structures: a bureaucratic education system extended by credential inflation to take up most of the young years; and the bureaucratic organizations (including the military) where large numbers compete for promotion through an elaborate series of ranks. Ironically, our age of meritocracy is more of a gerontocracy that the pre-bureaucratic era. The exception is young business entrepreneurs in Information Technology (because they don’t wait for credentials), although not in finance and management.
What makes the commanders of Buonapartist France seem so young for their rank is that the an exceptional event, the Revolution, freed the French army of many of its older ranks, many of whom were killed, fired, or driven into exile. A few of the Austrian generals Buonaparte faced in his 1796 Italian campaign were in their late 60s or 70s. Most were 50+.
But even age does not correlate with exceptional energy. Marshal Suvorov, whose matchup with Buonaparte is something military history fans still wish would have happened, led Russian troops across the Alps, won all his battles (including one at the Trebbia) and then led his troops back over the Alps in mid-winter because the Austrians stabbed him in the back. He was 69-70. While Arthur Wellesely, one of the two allied leaders who destroyed Buonaparte forever at Waterloo, was born in the same year as Buonaparte (and died a grand old man in 1852), his collegue Gephard “Marshal Forwards” von Blucher was 72. And what did Blucher do at his age, an age Buonaparte would never see: