A change in direction

Seeing as I am still in my 40th year and surely entitled to a mid-life crisis it has become apparent that I would like to follow a change in direction. That is not to say I will now enjoy the company of men rather than women or I will start smoking having lived a lifetime without feeling the need to fill my lungs full of smoke.
I was thinking more of a professional change.
This is of course easier said than done as I am not professionally qualified to do anything in life but I've gotten to where I am today by being able to learn quickly and adapt to situations. I would consider my present employment to be better than average and can say that I am quite lucky to be where I am but that is not to say that I am happy. I wonder how many more of us can attest to the same feelings? I wonder on the percentage of people performing the kind of job they always wanted and how many of us just settle?
I have been working now since the age of 17 and now that I am 40 that equates to 23 years gainful employment. If I am lucky enough to survive through to retirement age then I have another 25 years of work ahead of me. That is longer than I've worked up to this point in my life so why is it that opinion would state that you are too old at 40 to start another career?
There are obvious drawbacks in so much that I cannot leave fulltime employment to retrain as I have children and responsibilities to pay for so what then are my options? I could learn in the evenings and take three years or so to become trained in whatever subject I choose but that would only leave me trained and would not satisfy the endless line of employers requiring 10+ experience. Society is set up that it prefers to employ those that have settled on a career choice at the age of 18 and have sufficient experience by 28 to enable them to move onwards and upwards through grades and promotions.
Obviously then I am going to have to try and fly in the face of what society deems to be "normal" and will try, possibly in vain, to change direction by doing something I like rather than doing something I do. I have started this change by completing my second novel so we'll see where I get with this one. Unfortunately publishers prefer to deal with writers who have "experience" and who have previously been published. Where have I heard that before?
Society is wrong in so many ways and it feels good to try and buck the trend. It seems that if more people done it, the less unusual it would be but it is difficult to break free from what is considered normality. I will endeavour to keep you informed of my hopeful success.