The present inventors discovered that knockout mice whose S1-5 gene function is lost develop age-related diseases or symptoms. In such knockout mice, bone mineral content, bone mineral density, and bone strength were found to be decreased, and the number of osteoclasts in bone tissues was found to be increased. Analysis of osteoclast-forming ability using bone marrow cells derived from the knockout mice revealed that osteoclast-forming ability is enhanced and osteoclasts are larger in the knockout mice than in wildtype mice. When purified S1-5 protein was added to this in vitro system, osteoclast-forming ability was inhibited. Furthermore, administration of purified S1-5 protein to osteoporotic model mice showed that this protein has the effect of improving osteoporosis. The above findings demonstrate that S1-5 protein is useful for treating and preventing age-related diseases such as osteoporosis.