Surely, fractures are a danger of having thin bones--but it's not the break itself that makes osteoporosis so bad. It's what happens after the fracture.
A hip fracture, for example, doesn't age you, but the fracture triggers a chain of aging-related events. When you're bedridden, you become weaker, becoming more susceptible to infections. With less exercise, your arteries become less elastic and more prone to blockage. And finally, your immune system becomes more vulnerable to dangerous diseases and infections.