The present invention provides a method for measuring senescence of a test subject, including a step of measuring a level of taurine modification of mitochondrial tRNA Leu(UUR) in a biological sample isolated from a test subject, by a reverse transcription reaction from a primer with the tRNA Leu(UUR) as a template. In addition, the present invention also provides a senescence determining drug containing a primer containing a base sequence consisting of 10 - 25 bases and complementary to the mitochondrial tRNA Leu(UUR) . Furthermore, the present invention also provides a senescence determining kit containing a primer containing a base sequence consisting of 10 - 25 bases and complementary to the mitochondrial tRNA Leu(UUR) and a reverse transcriptase. Moreover, the present invention also provides an agent for improving a taurine modification rate of mitochondrial tRNA Leu(UUR) , containing taurine as an active ingredient.