I sometimes wonder about starting late in life in something new. But I'm convinced that you are as young as you feel and medical problems aside, you should never say "I can't" or "I'm too old".
Of course it's great to discover a new passion when in your prime, like in your early twenties or even before that. But it's never too late until they nail the lid down. And why shouldn't you be passionate and give it your all, just because your fourtieth (alternatively, insert ANY age) birthday came and went?
There's a great post and comments on Steve's BJJ Blog, titled Don't let ____ be your excuse.
So if you are out there wondering if you would like to try BJJ, Karate or whatever. Stop wondering and give it a try. Do it now.