A、on his fifty 应该是in his fifties 介词复数错误,意思：在五十几岁的时候、
B、at the age of fifty 译为“在他五十岁时”,强调在他50岁那一年发生的事情
C、when he fifty 本打算用时间状语从句,但是语法错误,应改为“when he was fifty”
D、in his fifties 在他五十几岁的时候