The Online Family answers:

A computer's lifespan depends a lot on how you want to use it and what upgrades are available. For example, your son can always enhance his laptop instead of buying a new one. He can upgrade his current computer's memory or operating system to improve its performance. Or if he's using it as a digital music library, then the hard drive could be upgraded to give him more space to store MP3s. He can also probably load up his laptop with the latest software that his friends have for creating and posting digital media online. However, generally speaking, an 18-month-old laptop could last him until graduation -- or beyond.