the down the shirt trick ...
Each time I get new rats, no matter the age, I take them out several times a day. I force them out, grab them and throw them down my shirt for an hour or so. Put on a t-shirt, and a sweatshirt and put the rats in between. Go read a book or watch tv and leave them there to bond to your smell. But... there are rats that for some reason don't ever become comfortable with their humans. But if you sit quietly, they will come to you and enjoy sitting on your shoulder. And to them, that's good enough, they are quite happy and content to have you talk gently to them and be their rat gym.