The question implies that at some level of density it is okay to fall of the bike hard.

I agree. if you are dense enough to deliberately fall hard from your bike, go for it.

Otherwise .. it is never okay to fall off your bike. Bone density is not an issue. You could fall wrong and break your neck or crush your skull, or in some other way kill yourself.

Art what level of density is That okay?

if you think you are too frail to ride, don't. But honestly ... what have you got to lose? So long as you have the balance and the awareness to keep from riding into things or the paths of moving things ... we are all leaving here one way or anther, at some time or another. if cycling makes you feel alive, ride your bike.

There is a thread in the Fifty-plus forum of a guy who wrecked hard ... he is getting a whole new hip. he will back and riding before long. plenty of people ride into their 80s. There are speed records for a rider who is about 106---he retired from competition not too long ago.

On the other hand, most people fall in the tub and break hips and worse.

Decide to live and live well. When your time comes, you will at least have made good use of your life.