If stored properly, even 6, or 7 years in storage is often fine. Tyres won't rot if stored properly. They might loose a bit of grip, but not too much.
This goes especially if the pavement where you ride has enough rocks in it, so that it is grippy. A guy I met from England explained how UK pavement is a lot less slippery then in our country, but the downside is that tyres get worn a lot faster.
Once a tyre is mounted, the "real" aging starts. Pressure, exposure to sun, cold, heating and cooling cycles. That is when tyres start to loose their attributes, even if they are not physically worn. So, depending on the motorcycle storing and riding conditions, after 2 to max. 5 years - replacement is needed. Sooner if they get worn, ofcourse. Or if a motorcycle is parked and left in direct sunlight, for days and months (especially in the summer).
Wrote at great length about
when should a bicycle tyre be replaced. It's not the same as with motorcycles, but some things are similar enough.
Though - Metzeller for example makes steel tyre casing (at least the models I've used), while Michelin and Continental for example, use some sort of fabric - that can be cut more easily and can rot more quickly.