Why does Hollywood always cast too old?
If the movie was about Napoleon's rise to power, well, he became "First Consul" at 30 and was crowned emperor at 35. Napoleon did is rising in his 20s and early 30s, and Joaquin Phoenix is now 48. By the time Napoleon was 48, he'd been defeated for the second time and was in exile on St. Helena, and only had a couple of years to live.
Hollywood always does this with reality-based films, casts an actor considerably older than the real person. At least, it does with male roles.