Middle-Age Here I Come!

I just read this article via Reuters, and found it to be a little deceiving. Nowhere in the article do they mention the age or the onset of middle age, "But the good news is that if people make it to aged 70 and are still physically fit, they are on average as happy and mentally healthy as a 20-year old." Should I assume then, that middle-age begins at 40 or 45? Can I claim this "middle age" persona, by feeling really depressed and craving something that lacks in my life?

The article says: 'For the average persons in the modern world, the dip in mental health and happiness comes on slowly, not suddenly in a single year,' Oswald said. 'Only in their fifties do people emerge from this low period.'" I agree, that exercising everyday and keeping a positive attitude will help with the whole aging process. What I don't agree with is the idea of middle-age. Can this mean, that I haven't reached this middle mark in life? Well, I'm pretty close, if 50 is the new beginning in life. Middle-Age; here I come. Oh, I found a few more gray hairs this morning and left the tweezers in the drawer.