Age evolution of the ratios of [Ne iii]/[Ne ii] (top) and [S iv]/[Ne ii] (bottom) for metallicities of Z = 0.4  Z _☉, Z = Z _☉, and Z = 2  Z _☉ from left to right. The Geneva Standard mass-loss evolutionary model with an electron density of n _e = 10 ² cm ⁻³ is used. The orange solid, magenta dotted, and red dashed lines indicate the ionization parameters of q = 2 × 10 ⁷ cm s ⁻¹, q = 8 × 10 ⁷ cm s ⁻¹, and q = 4 × 10 ⁸ cm s ⁻¹, respectively. The gray regions represent the range seen among the starburst-dominated GOALS sources.