Our rats are adopted out at 8+ weeks old. It is usually closer to 3-4 months though. It just depends on the rats themselves. This is because we want our rats to be both physically and mentally mature enough before they move on in their lives. While they may be weaned and separated from mom at 5-6 weeks old, they still rely on our adult rats to teach them body language, how to and how not to interact with other rats, how to interact with humans, etc. We also are monitoring these things. We observe all of their behaviors. This is so that we know if they will be a good fit in a pet home, and so that we can choose who will be staying here to continue their line.
Just those few extra weeks do a lot for these rats. They mature and grow so quickly. At 6 weeks old they just don't have the mental strength to handle such a big change. They don't know how to handle stressful situations like that yet. So we help them out and don't make them go through it. We care about each and every one of our rats and will do whatever it takes to give them the best lives possible.
Rats are so smart and understand affection and love. A rat will bond with you at any age. You will not get a better bond with an immature, scared young rat than you will a mature, confident older rat!