Why the same actor for the 60 year span?

Rating: 7/10 (27 November 2019)

First of all, why is Robert de Niro first choice for role of Frank "thr Irishman"? Maybe some resemblence wouldn't hurt, and real Frank was supposed to be a "mountain of a man".
Also 77 y.o. de Niro is playing this character since he was in II WW (20 year old soldier)? The scenes like him beating a guy on the street are simply not convincing. Why not have two actors to cover this time span 1944-2000.
This was a deal breaker for me.