That my friend, is a question people in their 50s have been asking themselves, in some form or another, for generations. As a 40ish year old, I am making that change a little early...but will probably still have to make another change in a decade or so...if I don't, I will consider it miraculous.
My husband did the same thing around 40 a few years ago.
My mother just made a huge career change...and she is 64. She loves it, and finds it invigorating.
In an ever changing world...It's all in your attitude. Stay flexible and never settle in too deep.
It suddenly occurred to me that I was 55 when my engineering job got outsourced. It was the best thing that ever happened to me. The solution is less government aid, not more.