A friend of mine has a nice Montesa that I've ridden a few times. It's pretty cool. You learn a lot about patience. The engine is essentially a CR250 Honda, but i think it requires race fuel.
There's still the potential for injury, but it's probably akin to rollerblading: scrapes and bruises mostly, but also a good chance to break your wrist.
I've actually been thinking about making him an offer on the thing, as he bought and never did anything with it. He was already in his 50's when he got it, so I guess it's something you could actually get into when you're older.
I have a long row of boulders that I moved to the back of my property and I can't help but think that it would be a perfect place to learn.