  We present a study of the abundance of Be stars in open clusters as a
function of the cluster age, using whenever possible ages determined through
Stromgren uvby photometry. For the first time in studies of this kind we have
considered separately classical and Herbig Be stars.
  The main results can be summarized as follows:
  1.- Clusters associated to emitting nebulosities and undergoing stellar
formation are rich in emission line objects, which most likely are all pre-main
sequence objects. No bona fide classical Be star has yet been identified among
them.
  2.- Clusters younger than 10 Myr and without associated nebulosity are almost
completely lacking Be stars, despite they have a complete unevolved B main
sequence.
  3.- Classical Be stars appear at an age of 10 Myr, and reach the maximum
abundance in the age interval 14-25 Myr.
  We interpret our results in the sense that the Be phenomenon is an
evolutionary effect which appear in the second half of the main sequence
lifetime of a B star. We propose that it can be related to main structural
changes happening at this evolutionary phase, which also lead to the recently
discovered non-monotonic helium abundance enhancement. The semiconvection or
turbulent diffusion responsible of the surface helium enrichment, coupled with
the high rotational velocity, can generate magnetic fields via the dynamo
effect and thereby originate the Be phenomenon. Observational tests to this
hypothesis are proposed.
