In the US OSHA has firm control over operator certification and a strict requirement for 3 year re-evaluations. They say nothing formally or informally in writing about trainers expiring in 5 years or any amount of years. You might find some compliance officer saying this or that in some part of the country, and consultants make statements like that all the time, but there is no document I have ever seen that says anything on the topic. Trainers that go through a class that has an expiration are getting that date due to one of two things. 1) The company doing the training wants to limit their liability to "X" years. 2) The company doing the training wants to get their money every "X" years. A trainer that is properly trained, stays current on any standard or interpretation changes and continues to train and stay active should not have a problem continuing to train indefinately. If they "want" to go back and get an update or refresher to include new materials are update themselves that is totally fine and maybe a good idea but it is DEFINATELY not required.