No baby rat should be away from its birth home before 5 weeks and preferably 6. The age at which you introduce a baby to an adult rat depends on the temperament of the adult. If you have just one or two adult rats which do not show much aggression to the baby, you may decide to introduce it as soon as you get it. If the baby is to be introduced to a large communal cage containing lots of rats, or if one of the older rats seems aggressive, then you may want to leave it a few weeks. On the other hand, some adult females consider baby-battering to be their favourite sport.
Babies are sometimes too fragile, too small, or just too silly to cope with an aggressive older rat.
But by 8 weeks and certainly 10 they should be able to defend themselves adequately without being so aggressive that they pick a fight with the older rat. Note that since male rats are often not aggressive to baby rats, it is better to try to introduce a baby boy to older boys as soon as possible and NOT to wait until he is old enough to stand up to him. But if the older boys seem vicious towards him then you may feel that it is better to postpone the introduction. These suggestions have been gleaned from various sources and numerous rat owners with far more experience than myself. Rats tell whether other rats are part of their family or social group by smell.