I took a complete change of career when I was 41, I left a relatively secure clerical job I had been in for 10 years to start up a franchisee business with my partner. Luckily it went well for me and 5 years later we are doing well (we run Subway sandwich stores). However, when we started the business, my partner stayed in his job (a well paid managment job) just in case it all came crashing down about our ears, we would still have had a fall back of his job to rely on. I think you need to consider what you would be giving up if you left your job i.e your salary, any pension or other benefits etc that you would lose. If you do not stand to lose too much and you have a good fall back plan in case things go wrong, then I would go for it and give it a try. Another thing you could try is to see if your present employer will give you some unpaid leave for a few months, you could then try out what you want to do, with the security of knowing that you have a job to go back to if it all fails.
The last thing I will add, is that we are heading into some uncertain times with the credit crunch taking effect etc, people may need to tighten their belts and spend less money, so you do need to be extra careful when planning to start a new business or new career.
Best of luck, Sue