Actually there is a difference."In his 50th year" means he has passed his 49th birthday, but has not yet had his 50th.(From birth to 1, you are in your 1st year, from 1yr. to 2yrs., you are in your 2nd year, from 49 to 50, in your 50th etc.) "50 years of age means he has passed his 50th birthday, but not his 51st.