  We have carried out a study of the early type stars in nearby OB associations
spanning an age range of $\sim$ 3 to 16 Myr, with the aim of determining the
fraction of stars which belong to the Herbig Ae/Be class. We studied the B, A,
and F stars in the nearby ($\le 500$ pc) OB associations Upper Scorpius,
Perseus OB2, Lacerta OB1, and Orion OB1, with membership determined from
Hipparcos data. We obtained spectra for 440 Hipparcos stars in these
associations, from which we determined accurate spectral types, visual
extinctions, effective temperatures, luminosities and masses, using Hipparcos
photometry. Using colors corrected for reddening, we find that the Herbig Ae/Be
stars and the Classical Be stars (CBe) occupy clearly different regions in the
JHK diagram. Thus, we use the location on the JHK diagram, as well as the
presence of emission lines and of strong 12 microns flux relative to the visual
to identify the Herbig Ae/Be stars in the associations. We find that the Herbig
Ae/Be stars constitute a small fraction of the early type stellar population
even in the younger associations. Comparing the data from associations with
different ages and assuming that the near-infrared excess in the Herbig Ae/Be
stars arises from optically thick dusty inner disks, we determined the
evolution of the inner disk frequency with age. We find that the inner disk
frequency in the age range 3 - 10 Myr in intermediate mass stars is lower than
that in the low mass stars (< 1msun). This indicates that the time-scales for
disk evolution are much shorter in the intermediate mass stars, which could be
a consequence of more efficient mechanisms of inner disk dispersal (viscous
evolution, dust growth and settling toward the midplane).
