Good question. We don't have studies to show the long-term results of each possible treatment option. Some may say don't worry about it -- what will be, will be. Others say don't borrow trouble before it happens. In other words, not everyone develops arthritis in a joint after a traumatic injury. The event increases your risk but doesn't guarantee it.
You didn't mention what treatment you've had for this problem now. Sometimes even young people damage the patella enough that fracture healing isn't possible. In those cases, the kneecap may be removed, a procedure called a patellectomy. Older folks might opt for a total knee replacement, especially if they already have arthritis in the knee. Middle-aged patients may be given the choice of just a kneecap replacement.
The biggest factor in long-term results may have to do with your knee alignment. Does the patella track up and down well during knee motion? Is it balanced and in the center? Does it tend to track more to one side or the other?
The knee joint axis is also important. Are you slightly knock-kneed or bow-legged? These positions can affect how well the knee holds up over the years.
Ask your doctor for his or her recommendations. It could be there's nothing to be done just now. Or there may be an exercise program that could correct any weaknesses or misalignments.