Typically with electronics, if you can make it past 90 days or so, they've passed the electronic burn in period and should last many years. In short, if something electronic is going to fail, it most often fails in the first few months. The things you have to worry about are (as Witeshark said) the batteries, and mechanical drives (optical, floppy & hard drive). As far as cosmetics are concerned (hinges, keys, etc), that all depends on individual manufacturer quality control. I still have a Wallstreet G3 Powerbook from '98 that still runs . . can't do much with it nowadays, but it still powers up
the average lifespan is considered 3 years. This is driven by business and taxes more than anything else. I have laptops that are more than 5 years old that work just fine. But after 3 years you have depreciated its value to the point where a business can no longer deduct any value from it.