Why is mid life the best time to make a career change?
For starters, by the time you hit "middle-age" you have built up a wealth of skills and experience on which to draw inspiration for new directions. Mr. Half Dome, whom I describe on my Mid Life Career Change page, spent his career doing sales and marketing presentations. Now he presents on a topic he knows and loves ! (and gets paid to do it)
By mid-life if you have children, they are probably old enough to take care of themselves, or out of your house completely, so the day-to-day parental responsibilities are gone. No longer constrained by kids at home, you have the freedom to choose what you WANT to do.
In mid-life you have the confidence and maturity that you might not have had when you were younger. You don't sweat the small stuff. Some fear agism and it would be naive to think that it doesn't exist. It does, in some fields more than others. High-tech for example does favor younger workers with cutting edge technology knowledge, but there are fields where experience is appreciated, such as health care, education and finance. See Best Employers for Mid Life Career Changers for some ideas.