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tokens: in the Upper Scorpius region, assuming only an age evolution from one region to another. Fig. 6. Comparison between median and percentiles of the mass accretion rates as a function of disk mass for the Lupus and Upper Scorpius regions ( left ), and for the Upper Scorpius region and the expectations from viscous models at 1 Myr and 8 Myr ( right ). 5.2. The impact of photoevaporation and variable accretion The observations are even more discrepant from models predicting the Ṁ acc − M disk relation by means of both viscous evolution and internal photoevaporation. As shown by Somigliana et al. (2020) , the effect of photoevaporation is the reduction in the number of accreting targets at low disk masses and mass accretion rates to an extent that, by ∼10 Myr, only a fraction of massive disks still survive. This is not observed here, where we see low-mass disks with high Ṁ acc . It is unclear whether this disagreement is due to the fact that the models assume only one stellar mass and two fixed mass-loss rates, or whether this is an issue of internal photoevaporation models in general. On the other hand, external photoevaporation would predict that the disks have low mass, while still low values of M disk / Ṁ acc ∼ 0.1 Myr (see Fig. 4 , and Rosotti et al. 2017 ; Sellek et al. 2020 ), more in line with what is observed here. In this context, it is worth mentioning that the environment of Upper Scorpius is different than the one of Chamaeleon I and Lupus, having more nearby massive stars (e.g., de Zeeuw et al. 1999 ). In such an environment, the effect of external photoevaporation could have been relevant for the evolution of disks, possibly more than dynamical interactions (e.g., Winter et al. 2018 ). Whether this effect has been dominant for the evolution of the disks observed here is still an open question. Further modeling is mandatory here, but it is nevertheless puzzling how the mass accretion rates can be retained for such long time with so little disk mass available. A possible solution to the fact that the accretion rates measured here are high given the measured M disk might be variable accretion. However, studies in younger star-forming regions have shown that, in general, typical variation of Ṁ acc are 0.4 dex in most disks (e.g., Costigan et al. 2014 ; Venuti et al. 2014 ), with only a small fraction of targets showing extreme variability of Ṁ acc > 1 −2 dex (e.g., Audard et al. 2014 ). We could imagine that the variability is larger in the old region of Upper Scorpius, but in this case we could expect a larger dispersion in accretion rates than in younger regions, which, however, was not observed. Further studies on variability in older star-forming regions is needed. The mass-budget issue given the observed Ṁ acc and M disk is therefore important. Indeed, under the assumption that Ṁ acc is constant with time, these high values of Ṁ acc would imply that over the lifetime of the disk 10 −8 M ⊙ yr −1 ×10 7 yr ∼ 0.1 M ⊙ of disk