Brown fat disappears with age so the earlier the better.  The dilemma you have is that rodents do not have great thermal control at younger ages.  You must be careful of any surgical procedures when exposing any rodent to cold.  With this said, you will have to very carefully  design experiments to "reduce" any pain or suffering that may occur.  Your IACUC (animal use committee) will demand this and ethics should as well.

Dear Peter 

Thanks for your opinion . Yes, i will get approval from the ethics committee for my experiment . 

with respect to the proper age of the mice for conducting cold exposure experiments , i found some literature in which they used "mature mice" ( 3-6 month of age ) for the same experiment .