Great question. I bought mine when it was around 5 years old (gasser), was a used work truck that was in good shape (unusual), higher mileage but I knew that I would not be adding on the mileage very much. For what it cost me I was able to have a number of upgrades done and it was ready to go, over the years I have done more upgrades and preventative maintenance and I know that I am way ahead than if I would have bought new.
There can be some REALLY good deals in the 3 - 5 year models as some people have traded in for a new vehicle and all that was wrong with the old one was it was not smelling new any more. Some times life changes and someone has changed jobs and now does not need the 3/4 ton, other times a kid or marriage comes along and out goes the truck.
My brother bought a $80,000 plus truck with all the toys from a young oil worker that realized that the payments that he had on the lease were nuts. The guy took a huge loss (over $40,000 counting what he had paid in down payment, payments, after market upgrades and the buyout penalty) just to get out of the lease, my brother bought a top of the line truck with all the toys and only a couple of years old for less than half price.
Don't rush and keep your eyes open.