I think that, though fifty is definitely an older Hobbit, we must remember that thirty-three was the Hobbit coming-of-age. Thirty-three to them was like eighteen to us. Their twenties (or "tweens"), was like our teens. So a fifty-year-old Hobbit is definitely going to look younger than a fifty-year-old human. Not to say that they were really young; they were probably equvalent to a human in their thirties.
As for the reason Tolkien chose this particular age, I really have no idea what he was trying to do, if in fact he was trying to do anything.
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