Global octopus catch has doubled over the past four decades and is likely to grow in importance as many key fisheries continue to decline. Accurate age data is critical to assessing the resource status of octopus. Over the past three decades, studies have investigated various methods to age octopus, with some methods better suited to certain species than others. However, only a small number of researchers have the hands-on knowledge to execute these methods in the laboratory. To address this knowledge gap, we created a comprehensive step-by-step guide to the octopus ageing process, as well as a decision tool, which should enable researchers to make an informed decision on the most suitable method for their species. Our guide consolidates existing methodologies and best practice for octopus aging, providing practical instructions for researchers new to the field or those working with previously unaged species. By publishing this foundational resource, we aim to enhance information accessibility and bridge the gap between specialised knowledge and practical application. Ultimately, we hope to promote the integration of age data into fisheries management to support the sustainability of octopus fisheries.