Model predicted intensity ratios as a function of burst age A_burst, for a hydrogen density of 10^2.8 cm^-3, and inner radius of 10^21.4 cm. The true burst age of these models is an average of A_burst and the older stellar population of 9.5 Myr, although the latter has a negligible influence on the high-ionisation lines considered here. The horizontal solid lines represent the observed ratios and the dashed lines the uncertainties on the ratios. The vertical dashed line shows the solution burst age of 3.7 Myr, and the vertical dotted lines the age of 3 Myr and 4.7 Myr that would better match the [Ne iii]/[Ne ii] ratio. (This figure is available in color in electronic form.)