I think in some regards, for some people, fifty is the new thirty. This would be particularly true of those that can afford good health care or have access to good health care. For the 1% in America, fifty is definitely the new thirty, but that wouldn't be true of the Americans that still can't access health care.
People are staying healthier longer, and lives are starting later and later because of longer education times and difficulty finding jobs. People are regularly settling down into careers in their mid to late twenties instead of earlier, making traditionally older ages, like 50, feel younger. Increased health means people have more years where they can act and feel young.
Those who say 50 is the new 30 have there heads stuck in the sand. At 50 you just cannot do what you could do at 30. You will not look the same, you will not think the same. Unless you're Tom Cruise.
^Check that out if you must.
No, 50 is not the new 30, because most people already have significant health problems at 50. People are living longer than ever before, but that does not mean that a person is still not heading towards old age by the time that they are 50. The body has broken down and they are in the last part of their career.
Although people can still be incredibly attractive at 50 I don't believe 50 is the new 30. Our life expectancy hasn't increased by 20 years and I doubt it will in the near future. Our bodies break down and a healthy 30 years old is still going to be in better shape than a health 50 years old.