Sarah died in 1966 and Richard's wife now said that William was 9 years old. But when William talks to her father, he said that he's 50 now. So he was born in 1957 and the contemporay time of the movie is in 2009 so he suppose to have 52 years old...not 50.
Maybe the end scene with Julia and William in the restaurant in New York was set in 2009, which was two years after Julia visited William in Florence. The scene where William talks to his father and mentioned his age as 50 happened shortly (several days, maybe a few weeks) after Julia had visited him in Florence.
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This sounds correct to me. Although I think the OP is being maybe too literal. I'm in my 50's but can see myself saying, "Now that I'm 50...." meaning "Now that I hit the 50 mark...." It doesn't mean right at this moment I hit the 50 mark. It means, now that I'm passed age 50.