$V_{\rm med}$     $\alpha$         $\beta$      $V_{\rm med}$     $\alpha$         $\beta$      $V_{\rm med}$
                                               (kms$^{-1}$)                                    (kms$^{-1}$)                                    (kms$^{-1}$)
   NGC 628    $1.18\pm0.13$    $5.40\pm0.27$    $3.0\pm1.9$    $0.95\pm0.09$   $5.83\pm0.20$    $1.6\pm0.8$    $0.59\pm0.19$   $6.68\pm0.42$        ...
   NGC 3184   $1.03\pm0.27$    $5.06\pm0.55$    $8.3\pm1.1$    $0.90\pm0.16$   $5.20\pm0.35$    $7.6\pm6.3$    $0.91\pm0.36$   $5.20\pm0.83$        ...
   NGC 3726   $-0.40\pm0.32$   $7.50\pm0.65$        ...        $1.22\pm0.17$   $4.37\pm0.37$     $29\pm25$     $2.04\pm0.26$   $2.38\pm0.62$        ...
   NGC 5585   $1.56\pm0.46$    $4.09\pm0.97$        ...        $1.41\pm0.27$   $4.38\pm0.62$        ...        $1.40\pm0.45$   $4.39\pm1.05$        ...
   NGC 6946   $1.01\pm0.14$    $6.00\pm0.27$   $0.98\pm0.62$   $0.76\pm0.10$   $6.46\pm0.21$   $0.54\pm0.27$   $0.38\pm0.22$   $7.35\pm0.48$        ...
  ---------- ---------------- --------------- --------------- --------------- --------------- --------------- --------------- --------------- ---------------

Unfortunately, different modern evolutionary synthesis models do not provide an unambiguous dependence between colour and age. As an example, we present two evolutionary models in Fig. \[figure:ubt\]. The first of them was described in Section \[sect:select\] (CMD), the second model was calculated using PEGASE.2[^5] code [@fioc1997]. Both models are based on the Padova stellar library, both have the same parameters of stellar population (SSP, Salpeter IMF with the range of masses from $0.1 M_{\odot}$ to $100 M_{\odot}$). Fig. \[figure:ubt\] shows the similar behaviour of evolutionary tracks, however we observe the offset $\approx0.2$ mag between tracks of CMD and PEGASE models.

Given the uncertainties associated with the choice of a model, unknown metallicities of star clusters, and observational errors, we use a linear 