I think slug damage is the biggest problem I've had growing any of the squashes. Until the plant is well established slugs seem to be able to kill them off. Try using slug pellets, if you've no objection to them.
I've had more success with Festival, or even Crown Prince, than Butternut down here in the deep south.
I think it depends on the race. The race promotor can set any age limits, and age groups, he likes, as long as it doesn't break the rules on maximum distances permitted - eg over 11 for 5k, over 15 for 10k etc.
The RD can also award prizes based on 5 or 10 year groups as he sees fit.