Definitely not the oldest cars regularly used in passenger service in the US or Canada -- though they're the oldest cars regularly used in *commuter* service.
They might also be the oldest with relatively intact original interiors, depending on how intact the interiors are. These cars were manufactured in 1954.
Most of the Amtrak Heritage diners are older, with the earliest dating to 1947 IIRC. (Interestingly, the Heritage diners are Budd, stainless, and many of the survivors were originally ordered for the CB&Q.) I'm fairly sure that a bunch of Amtrak's baggage cars are older, too, though railfans don't seem to keep careful track of those.
The consists of antiques which VIA Rail uses include lots of pre-1954 cars, and at least one coach which is the most likely candidate for "oldest car currently in regular passenger service". VIA Rail is using a 1951 National Steel baggage car on the Winnipeg-Churchill train. The Galley Club cars (used on "VIA 1") are from 1947-49. But VIA is also running coaches built as early as *1946*. And these are on the "Corridor". Even with the recent ax taken to what's left of VIA services, the 1940s cars are still going to be running in daily service for a while yet.