That is some exceptional mileage for any rim... a rim like the Mavic MA40 which was renowned for it's high mileage and lifespan and one could expect 15,000 miles on one of these under hard use like touring.
If the OP's were on road bikes that saw nice weather and minimal braking getting 20,000 miles is feasible... their degree of wear is going to be determined by the braking surface and it's condition and not if they have developed cracks which is an issue that stems from material failure or a really poor build.