Context : There are a variety of different techniques available to estimate the ages of pre-main-sequence stars . Components of physical pairs , thanks to their strict coevality and the mass difference , such as the binary system analysed in this paper , are best suited to test the effectiveness of these different techniques . Aims : We consider the system WW Psa + TX Psa whose membership of the 25-Myr \beta Pictoris association has been well established by earlier works . We investigate which age dating technique provides the best agreement between the age of the system and that of the association . Methods : We have photometrically monitored WW Psa and TX Psa and measured their rotation periods as P = 2.37 d and P = 1.086 d , respectively . We have retrieved from the literature their Li equivalent widths and measured their effective temperatures and luminosities . We investigate whether the ages of these stars derived using three independent techniques , that is based on rotation , Li equivalent widths , and the position in the HR diagram are consistent with the age of the \beta Pictoris association . Results : We find that the rotation periods and the Li contents of both stars are consistent with the distribution of other bona fide members of the cluster . On the contrary , the isochronal fitting provides similar ages for both stars , but a factor of about four younger than the quoted age of the association , or about 30 % younger when the effects of magnetic fields are included . Conclusions : We explore the origin of the discrepant age inferred from isochronal fitting , including the possibilities that either the two components may be unresolved binaries or that the basic stellar parameters of both components are altered by enhanced magnetic activity . The latter is found to be the more reasonable cause , suggesting that age estimates based on the Li content is more reliable than isochronal fitting for pre-main-sequence stars with pronounced magnetic activity .