It's either because of fear of death or fear of not looking as desirable as you did in your twenties and thirties. I definately think the media plays into this age paranoia. Our society in general does not revere elderly people, as some Eastern cultures do. In Japan, elders are revered & appreciated for their wisdom. Not so in the Western world.

I have to say that I did get very upset when I turned 50. Now that I am 51, it's not as much of a big deal. It's going to happen anyway so why fight it?

Interesting topic, thanks for posting.