There are several methods that can be applied to dead individuals or museum specimens (skeletons). In the latter case, the analyses of tooth wear or baculum (os penis) in carnivores, for instance. The analysis of the dry weight of the eye lens or the analysis of the growth of the long bones are others.

This review is old, but recent literature on this topic, as to my knowledge, is scanty: 

Morris, P. (1972). A review of mammalian age determination methods. Mammal review, 2(3), 69-104. 

 Here you can find some methods of age estimation for the rat:

The Laboratory Rat: Relating Its Age With Human's, Sengupta, 2013 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3733029/)

Dear Mahmoud

Please, use the following link:

https://www.degruyter.com/view/j/mamm.2002.66.issue-2/mamm.2002.66.2.275/mamm.2002.66.2.275.xml

Good Luck

look; Hafermehl K. H., 1975: Altersbestimmung bei Haus- und Labortieren. 2.ed.

Paul Parey, Berlin und Hamburg.

Habermehl, K. H. , 1980: Die Altersbestimmung von Versuchstieren, (Age determination of laboratory animals). Paul Parey, Berlin und Hamburg