Originally Posted by Simurgh
When you are old and well past your prime.
If you're old and past your prime you should retire. If you're still considered one of the best you should be fighting other top rated fighters.
The only time its ok to duck a top challenger is when you've already beaten him as champion and there are stronger fighters to fight. For instance, there was no need for the second Wlad-Thompson fight. I would have been fine with Wlad taking on another top 10 contender.