It really amazes me how "old" some younger people think that 50 is! My first "midlife crisis" was at age 25!
That's the only time I remember feeling depressed about a birthday, because I would no longer be in my early 20's.
When I was in my mid-40s, I was beginning to feel old. I had gained weight and was having knee problems. I was dressing in matronly clothes and making a mental list of "things I would never do again." Then I held my grandmother's hand as she died on her 85th birthday and it dawned on me that if I lived as long as she did, I had only lived about half my life. Was I going to spend half my life as an "old" person?
So I began to change my attitude. I lost weight, became a blond, got a master's degree, bought a Saleen Mustang convertible, and now I ride a bike. I'm not pretending to be a kid -- I'm acting my age as far as I'm concerned and having a blast!