As I don't drive much in the winter, my winter tyres still have good tread after five seasons here, so I'm wondering if tread is the only criteria for changing to a new set of winter tyres?

I seem to remember reading something about this but can't track down the relevant thread among all the other winter tyre threads. Do they have some kind of "use by" concept?

5 years after manufacture is about the useful life as the rubber gets harder.
If you don't drive much in the winter with snow on the road you might be OK with all season tyres Pierelli do a P6 version that's quite good.

As FMF says, around 5 years. Winter tyres contain a richer content of silicone and usually stay softer for longer than a normal summer tyre. If in doubt, take a good look at the tyre tread. Look for small/hairline cracks in the rubber between the treads in the grooves. If you cant see any, then most likely they will be OK for light use.

If you look up old tyres on google images, you can see what I mean. Here is an example of a tyre with tiny cracks in the grooves. This means it is **cked!

Five millimeter tread depth at the beginning of the season is a general rule of thumb for winter tires. Match up your five years and five millimeters to determine if your tires are any good.
JC