1. A software aging test system comprising: a circuitry configured to: execute load test of a version of software under test in accordance with an execution criterion; detect presence or absence of a software aging problem by comparing a test result of the load test with a test result of load test of a previous version of the software to be compared; update the execution criterion so as to change frequency of execution of subsequent load test based on a result of the detection; obtain an amount of change quantitatively indicating a difference between a software version under test and a latest stable version of software, the latest stable version of software being free of a software aging problem; determine, from the amount of change, whether or not a load test is necessary to be executed based on the execution criterion; execute load testing of the software version under test when an execution of the load test is determined as necessary by the determination; detect presence or absence of a software aging problem by comparing a test result of the load test of the software version under test with a previously obtained test result of load testing of the latest stable version of software; and update the execution criterion so as to change the frequency of the execution of subsequent load test based on a result of the detection of the presence or absence of a software aging problem.