The frequencies of TDs (diamonds) and classical TDs (filled circles) among the disk population in different clusters/aggregates, including NGC 1333, L1688, NGC 2068/2071, IC 348, OB1b, η Cha, OB1a/25 Ori. The observational data are obtained from Muzerolle et al. (2010), besides the age of NGC 2068/2071 which is from Fang et al. (2009). The left-pointing arrows indicate that the ages for NGC 1333, and L1688 are up-limits. The TD sample include classical TDs and weak excess TDs from Muzerolle et al. (2010). The dashed line is the predicted frequency () of TDs among all the disk population as a function of age. The solid line is the predicted frequency (f_CTD  =  0.44 f_TD) of classical TDs among all the disk population as a function of age.