Until someone in the industry corrects me (which they might have to do), there is no industry-wide rule. When game shows were mostly network propositions instead of syndicated, the policy of a given show was whatever the network's was. And (IIRC, and I may well not), syndicated offerings of that time sometimes followed the policy the station they originated from (a champion forced to retire on syndicated TJW being a good example).
These days, it's pretty much piecemeal. GSN offerings are boilerplate, but everything else it all over the map.
To answer the other question, from the PiR FAQ:
"Contestants must be 18 yrs. of age and not have appeared on The Price Is Right before."
edit: well crap, that's what I get for answering the doorbell in the middle of a reply.