Dynamical age as a function of the mass (bottom axis) and as a function of initial density (top axis) of RCW 120 (blue dot), with the uncertainty associated to each of them (blue bars) and the linear fit (purple line). The fragmentation timescale following the model of Whitworth et al. (1994) is plotted for N_LyC = 10⁴⁸ s⁻¹ and c_s = 0.3 (red squares), and for c_s = 0.2, 0.4 km s⁻¹ (green dashed lines).