^^^^^
This
Of course I encourage people to sex their animal upon arrival this way if a mistake is made it can be corrected immediately, however as Ed said, sex won't change whether it is 6 month or 2 years down the road and therefore the mistake should be corrected regardless of the time frame that went by.
For those that think after 2 years it's unreasonable to expect compensation, if hets don't prove after 2 or 3 years YOU or anyone would expect compensation right? Well sex is the same thing.