Summarize this article:

If you have your choice and are looking to interact with your rat or introduce a new rat to your current rats, pick one that is under 6 months old. After this age, they begin to develop their social behaviour and if they live in the wild or without much human contact, that is what they'll be used to.. That said, do not overlook older rats that are in need of a new home! All rats are wonderful, some just may take longer to be comfortable in their new surroundings. Older rats can be socialized with the same methods but you'll need to be more patient as it will take them longer to trust you. Rats will come to trust your hands if they associate them with food. Start by hold the food with your fingertips. After you've fed it from your fingertips a few times, hold the food in the palm of your hand and let the rat sit in your hand as you eat. Eventually, it'll instinctively walk onto your hand anytime you hold it out. Rats, much like dogs, have the ability to differentiate voice patterns. Try to repeatedly say its name softly over time. It may learn to remember its name and come to you when you say it..Remember to reward your rat for coming to you when called with a small piece of a snack. Because a rat's eardrums are so small, it won't be able to differentiate sounds that are too loud. Try whispering with your mouth close to its ear. Never squeeze your rat and avoid restraining it for more than a few seconds at a time. Keep your hands open and let the rat move around at will. By spending time in your hands, your rat will get used to your scent and bond with you. Pick up your rat by placing your hand under its belly. Try to use two hands when picking it up. Putting your rat on its back so that its belly is exposed is a sign of dominance and can be stressful to your rat, especially when there is no real reason at that particular moment for you to be asserting dominance. Once it's used to being held, try to simply hold your hand out and let the rat come to you.

Summary:
Like dogs and cats, rats are much easier to socialize when they're younger. Offer treats from your hand. Speak softly. Hold your rat correctly.