I've a 5 year old laptop and never had an issue with it. In fairness, they get more battering then a desktop due to being carried around and left lying around. as for an inherent lifespan, to some degree, everything has planned obsolescence built in to some degree.
I think you are asking the wrong question though, you should be asking what are you going to use the PC for. if it is for work and you need to sit at a desk and don't need to travel with a computer then consider a desktop, if you are looking at surfing the internet or using it whilst lounged on the couch or if you need it on the move, then a laptop is the choice