This Research Note presents the findings of a study which examined how biodiversity changes with stand age, with a view to incorporating it into optimal forest rotation length modelling. The study reviewed relevant literature and analysed Forestry Commission Biodiversity Assessment Project data. The review revealed no simple or universal response of biodiversity to stand age. However, there was more evidence of biodiversity increasing with stand age than falling (or not changing) and, with regard to habitat requirements for birds and mammals in British forests, there is evidence that after a brief initial increase, biodiversity declines until around 20 years and thereafter increases again. While only a limited number of economic models were found which linked biodiversity and rotation length, two distinct approaches to such work were identified: first, a direct approach which accounts for biodiversity values when estimating net present values and, second, an indirect approach which employs biodiversity management constraints in the modelling. The data analysis also revealed, in most cases, no evidence of significant changes in biodiversity with stand age. Upland Sitka spruce stands were an exception, where biodiversity levels were higher in young forests and again in more mature forests and at a minimum at around 40 years old. Overall, the study found that both the ecological evidence linking biodiversity and stand age and the economic modelling accounting for that linkage are limited. Therefore, a substantial challenge remains to incorporate biodiversity into rotation length models, and recommendations are made to address this.