I have a cousin who is a volunteer firefighter in Ravalli County, MT. He told me that their oldest truck is a 1974 Ford brush truck on a pickup chassis similar to this one. http://media.liveauctiongroup.net/i/7443/9197799_1.jpg?v=8CC87BD44FFA290
It's certainly possible that smaller areas would have trucks a decade older. I live in Los Angeles, our neighboring county agency, Orange County Fire Authority, has a 1970 Crown foam tender that they keep in active service.