Thus far our impressions regarding the evolutionary time scales for young circumstellar disks have been based on small number statistics .
Over the past decade , however , in addition to precision study of individual star/disk systems , substantial observational effort has been invested in obtaining less detailed data on large numbers of objects in young star clusters .
This has resulted in a plethora of information now enabling statistical studies of disk evolutionary diagnostics .
Along an ordinate one can measure disk presence or strength through indicators such as ultraviolet/blue excess or spectroscopic emission lines tracing accretion , infrared excess tracing dust , or millimeter flux measuring mass .
Along an abscissa one can track stellar age .
While bulk trends in disk indicators versus age are evident , observational errors affecting both axes , combined with systematic errors in our understanding of stellar ages , both cloud and bias any such trends .
Thus detailed understanding of the physical processes involved in disk dissipation and of the relevant time scales remains elusive .
Nevertheless , a clear effect in current data that is unlikely to be altered by data analysis improvements is the dispersion in disk lifetimes .
Inner accretion disks are traced by near-infrared emission .
Moderating a generally declining trend in near-infared continuum excess and excess frequency with age over < 1 to 8 \pm 4 Myr , is the fact that a substantial fraction of rather young ( < 1 Myr old ) stars apparently have already lost their inner accretion disks while a significant number of rather old ( 8-16 Myr ) stars apparently still retain inner accretion disks .
The age at which evidence for inner accretion disks ceases to be apparent for the vast majority ( \sim 90 % ) of stars is in the range 3-8 Myr .
More distant , terrestrial zone dust is traced by mid-infrared emission where sufficient sensitivity and uniform data collection are only now being realized with data return from the Spitzer Space Telescope .
Constraints on mid-disk dissipation and disk clearing trends with radius are forthcoming .