Fig. \[figure:hist628\], show similar shapes for pairs with $\log \Delta S\ge2.8$. However, the distribution of close cluster pairs ($\log \Delta S=2.0$) differs significantly from them. A relative deficiency of cluster pairs with a large age difference is observed clearly for pairs with $\log \Delta S=2.0\pm0.1$ (the red shaded histogram in the figure).

On large spatial scales ($\log \Delta S>2.4$), distributions of cluster pairs reach a maximum $N_{\rm max}$ at $\log \Delta t \approx 7.5$ and have a flat profile for the pairs with $\log \Delta t = 7.5-8.4$. Note that $\log N_{\rm max} \sim \log \Delta S$ (Fig. \[figure:hist628\]). These facts indicate a significant proportion of cluster pairs with $\Delta t \sim 100$ Myr, randomly and evenly distributed on spatial scales of a few hundred pc and more. On smaller scales ($\le100$ pc), the existence of cluster pairs with large age difference is difficult. Starburst at $\sim100$ pc scale has a duration of $\sim10$ Myr [@efremov1998], it destroys H$_2$ cloud remnants. Star formation here becomes impossible for at least 100 Myr (ISM relaxation time).

Stationary density wave theory predicts the existence of an age gradient across the spiral arms at least in grand-design galaxies [see survey in @dobbs2014]. Observational studies 