It depends. My 5-year-old macbook (1st gen of plastic white macbook) is still running strong. I bought an Air last month but keep it around as a back up machine

It depends. My 5-year-old macbook (1st gen of plastic white macbook) is still running strong. I bought an Air last month but keep it around as a back up machine. I replaced the battery 2 years ago though, just before the warranty expires and somehow they did it for free.