I actually just had this discussion with my vet since I was asking her how long I should continue the 18% protein food. She said sort of the same thing but that it can vary rat to rat. Some stop growing sooner than others. She said that as long as they are gaining weight regularly, consider they are still growing and keep them on the higher protein food. Once they stop growing, their weight will stabilize and remain fairly constant. That's when you know that they are done growing.It depends a lot on the rat and its sex. Boys tend to stop length wise about 6 to 9 months but are still filing out muscle wise at 12 months plus. Girls tend to be a bit earlier, getting there full length at around 5 or 6 months then finish filling out around 7 or 8 months.