Line flux of ¹²CO (J=3-2) scaled to 100 pc as a function of the stellar age. The pink points show ¹²CO (J=3-2) fluxes measured in this work, the green points show archival ¹²CO (J=3-2) fluxes, and the purple points show archival ¹²CO (J=2-1) fluxes that were scaled by the average ¹²CO J=3-2/J=2-1 ratio of all discs for which both lines were observed (1.67). The archival sources and the corresponding references are listed in Table E.1. The circles represent resolved data points, and the triangles represent upper limits on ¹²CO.