Tryptophan is an essential nutrient required to produce niacin (vitamin B3), which is needed for energy metabolism and DNA production. Alterations in tryptophan metabolism could have significant effects on aging and musculoskeletal health. The kynurenine pathway, the main catabolic route of tryptophan, is modulated by inflammatory factors, which are increased in older persons, a process known as inflammaging. Osteoporosis, sarcopenia, osteosarcopenia and frailty have also been linked with chronic increased levels of inflammatory factors. Due to the disruption of the kynurenine pathway by chronic inflammation and/or changes in the gut microbiota, serum levels of toxic kynurenines are increased and have been associated with osteoporosis, sarcopenia and frailty. In contrast, anabolic end products of this pathway, such as picolinic acid, have demonstrated a beneficial effect on bone and muscle. In addition, recent studies have shown that physical activity can modulate this pathway by promoting the secretion of anabolic kynurenines. According to the evidence collected, kynurenines could have a promising role as biomarkers for osteoporosis sarcopenia, osteosarcopenia and frailty in older persons. In addition, some of these metabolites could become important targets for developing new pharmacological treatments for these conditions.