There are some spammers out there who register a domain for a short period of time, misuse the service and at the end of the registered period, they no longer need the website. That is one reason why Google thinks it is necessary to check the domain age. A legitimate domain user will register in advance for a longer period of time. There are a lot of arguments about this particular ranking factor, there are varied opinions too. Some say that it should not at all be a ranking factor, some say it should be, and yet some think that it should be a really minor factor.