Well there is sexually mature and than there is optimal for breeding.

By age 5 weeks rats are sexually mature.

The optimal breeding weight for females is around 250 grams, I really don't go by age I simply start pairing them when they reach that weight, takes a while to reach that weight but definitely not 4 to 5 months , usually by 2 months old the females have reach 250 grams.

Males well you have to use common sense at 5 weeks they are ready however if the size difference between the male and the female is too big the male is just gonna get beat up.