  In previous works of this series, we have shown that late B- and early A-type
stars have genuine bimodal distributions of rotational velocities and that late
A-type stars lack slow rotators. The distributions of the surface angular
velocity ratio \Omega/\Omega_crit (\Omega_crit is the critical angular
velocity) have peculiar shapes according to spectral type groups, which can be
caused by evolutionary properties. We aim to review the properties of these
rotational velocity distributions in some detail as a function of stellar mass
and age. We have gathered v sin i for a sample of 2014 B6- to F2-type stars. We
have determined the masses and ages for these objects with stellar evolution
models. The (Teff, log L/Lsun)-parameters were determined from the uvby-\beta
photometry and the HIPPARCOS parallaxes. The velocity distributions show two
regimes that depend on the stellar mass. Stars less massive than 2.5 Msun have
a unimodal equatorial velocity distribution and show a monotonical acceleration
with age on the main sequence (MS). Stars more massive have a bimodal
equatorial velocity distribution. Contrarily to theoretical predictions, the
equatorial velocities of stars from about 1.7 Msun to 3.2 Msun undergo a strong
acceleration in the first third of the MS evolutionary phase, while in the last
third of the MS they evolve roughly as if there were no angular momentum
redistribution in the external stellar layers. The studied stars might start in
the ZAMS not necessarily as rigid rotators, but with a total angular momentum
lower than the critical one of rigid rotators. The stars seem to evolve as
differential rotators all the way of their MS life span and the variation of
the observed rotational velocities proceeds with characteristic time scales
\delta(t)\sim 0.2 t_MS, where t_MS is the time spent by a star in the MS.
