You can swap out the tires after 18 months if you have money to burn...
It certainly depends how much driving you do and what conditions the roads are in - everything from the temperature, road surface, amount of turns and stops, etc. make a difference.
On my car, a set of tires (winter or summer) won't usually last more than 2 seasons anyway. I drive around 30'000 KM per year, and a set of European tires will usually not last much more than that. (Yes, when I was living in California, we had 50'000 mile tires, but we don't have any winter in Southern California!).
On my motorcycle, I usually end up replacing the tires after about 6500 km - a normal season for me. Sadly, I didn't get much riding in last year and have only been out twice this year so far, so my tires may hold into 2013
As far as age goes, I wouldn't ride a motorcycle if the tires are older than 5 to 6 years because bike tires must be much softer and will degrade much faster. For a car, tires that are 6 or 7 years old are still OK, but I would get worried if they were 8 or more years old and had been exposed to the elements (sunlight/UV) for a long time.