Context : Stars show various amounts of \gls rv jitter due to varying stellar activity levels .
The typical amount of \gls rv jitter as a function of stellar age and observational timescale has not yet been systematically quantified , although it is often larger than the instrumental precision of modern high-resolution spectrographs used for Doppler planet detection and characterization .

Aims : We aim to empirically determine the intrinsic stellar \gls rv variation for mostly G and K dwarf stars on different timescales and for different stellar ages independently of stellar models .
We also focus on young stars ( \lesssim 30 Myr ) , where the \gls rv variation is known to be large .

Methods : We use archival FEROS and HARPS \gls rv data of stars which were observed at least 30 times spread over at least two years .
We then apply the \gls pv technique to these data sets to identify the periods and amplitudes of underlying , quasiperiodic signals .
We show that the \gls pv is a powerful tool to identify quasiperiodic signals in highly irregularly sampled data sets .

Results : We derive activity-lag functions for 20 putative single stars , where lag is the timescale on which the stellar jitter is measured .
Since the ages of all stars are known , we also use this to formulate an activity–age–lag relation which can be used to predict the expected RV jitter of a star given its age and the timescale to be probed .
The maximum \gls rv jitter on timescales of decades decreases from over 500 m/s for 5 Myr-old stars to 2.3 m/s for stars with ages of around 5 Gyr .
The decrease in \gls rv jitter when considering a timescale of only 1 d instead of 1 yr is smaller by roughly a factor of 4 for stars with an age of about 5 Myr , and a factor of 1.5 for stars with an age of 5 Gyr .
The rate at which the \gls rv jitter increases with lag strongly depends on stellar age and reaches 99 % of the maximum \gls rv jitter over a timescale of a few days for stars that are a few million years old , up to presumably decades or longer for stars with an age of a few gigayears .

Conclusions :