I purchased a "new" bike from my local bike shop that is five years old--should they have disclosed this?
If it's actually used, as in previously owned with real mileage, they should have disclosed that. If it's in new condition, or new condition with some amount of the discount being for shop wear or due to being a low-mileage return, then I would say there's nothing disingenuous about omitting the manufacture date unless asked directly.

For better or for worse, it's not the standard practice in bike shops to include the model year on the sticker, as is seen for example on a car lot. Maybe it's a reasonable question whether that makes the whole industry that much scummier, but following the practice doesn't make any particular shop especially scummy.

Many who work at or run shops don't have great feelings about the industry's general insistence on model year cycles. The way it tends to play out is the model year cycle is one of the back industry's tools to generate sales based on perceived obsolescence, whereas we in shops know if a bike was good and worth standing behind 5 years ago, it very probably still is today. Meanwhile, however, the model year cycle creates a need in the retail environment to constantly worry about selling through stock before it becomes last year's model, an artificial panic which really doesn't do the retailer any good in the end. This is only to give some background about common attitudes in shops on the topic.