OK, I'm over 45, way over. I ride MTBs within my means, (As Clint Eastwood said, a man's got to know his limitations). If at all possible, I stay away from rock gardens. If I know the trail, I ride it as fast as I can. If I don't know it, I ride it at a moderate pace i.e. slow. Yeah, I still have the occaisional fall, its going to happen and I don't worry about it. If I'm tired, its time to head home, cause that's when I have falls.
As for major falls that result in significant medical bills. Yeah, I've had two, both on my road bikes. I lost count of the number of times I fell on my MTBs, but none resulted in a visit to the doc.
I never thought of dirt as a soft, then I fell on asphalt, no broken bones, but stitches, shoulder injury, operation and so on. So before you start riding road because you want to avoid the injuries associated with mountain biking, check how hard a roadway is, then check the trail you're riding. I still ride both, but the truth is, I worry less about falling on the MTB than the road bikes.