There's a lot to protect our Boxers from, no matter their age. And while not all random accidents cannot be prevented, there are several steps you can take to drastically lower the chances of your Boxer suffering a broken leg:
1. Do not allow your Boxer to be off leash outside. With being hit by a passing car the #1 reason for receiving broken bones, and there is of course a possibility of a fatal accident, take steps that your Boxer cannot take off running off leash.
2. With jumping from heights & sporting events (Frisbee, etc.) the next top causes, it's up to you to supervise your Boxer in regard to what he can and cannot handle.
3. Since rough play is the 4th top cause, don't assume that two dogs will automatically play together with the exact right amount of force and restraint. It's really easy for dogs to be having fun and get carried away.
In multiple dog households, while you do want to encourage your dogs to be friends, it's best to offer very close supervision until the dogs are either close in size or have proven themselves to show control in regard to enthusiasm.