I think 6 weeks would certainly be possible but it might be stressful to the baby. There are breeders that routinely pull at that age or even older, but they generally do some handling of the babies in the nest starting earlier, so the babies are not frightened of them. Once their eyes are open and they can recognize their parents, it's scary to switch. Older babies may also be fussier with a change in diet as well.
Is the person offering the baby located near you? You might see if you can visit this baby a little, it may be possible to work with it a little while it's in the nestbox to help gain its trust early on, and then bring it home as soon as it's weaned. Generally the only real difference between a handfed baby and a tamed parent raised baby is that handfed babies are more fearless since they get exposed to a lot more people and new things while being handfed. So with a parent raised baby, you really want to focus on a lot of socialization and exposure to new foods, toys, people, ect. in the first few month after weaning. But that's a good idea with handfed babies as well.