Try this, it builds off of AKV's answer but uses your code and contains the for loop. I think this should work!
NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"age" ascending:YES];
NSArray *sortedArray = [myNsArrayController.arrangedObjects sortedArrayUsingDescriptors:[NSArray arrayWithObject:sort]];
for (animal *a in sortedArray)
{
// Rest of code
}