  We study the time dependency of Vega-like excesses using infrared studies
obtained with the imaging photopolarimeter ISOPHOT on board of ISO. We review
the different studies published on this issue, and critically check and revise
ages and fractional luminosities in the different samples. The conclusions of
our study differ significantly from those obtained by other authors (e.g.
Holland et al. 1998; Spangler et al. 2001) who suggested that there is a global
power-law governing the amount of dust seen in debris disks as a function of
time. Our investigations lead us to conclude that (i) for stars at most ages, a
large spread in fractional luminosity occurs, but (ii) there are few very young
stars with intermediate or small excesses; (iii) the maximum excess seen in
stars of a given age is about a fractional luminosity of approximately 10^{-3},
independent of time; and (iv) Vega-like excess is more common in young stars
than in old stars.
