Human astroturfers can then be assigned these "pre-aged" accounts to create a back story, suggesting that they've been busy linking and retweeting for months. No one would suspect that they came onto the scene for the first time a moment ago, for the sole purpose of attacking an article on climate science or arguing against new controls on salt in junk food.
http://www.guardian.co.uk/environment/georgemonbiot/2011/feb/23/need-to-protect-internet-from-astroturfing