Many classes have a "minimum wind speed" - the Hobie classes use 5 kts; the Tornados use 6 kts.

If they are used, however, they are almost always specified in the SI's. The PRO should have read the SI's for the event (I always have a set with me on the signal boat).

I don't know of any class that uses "5 mph". Still, it's the PRO's perogative to start or not start a race.